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CHAPTER

39BONE GRAFTS AND IMPLANTS

IN SPINE SURGERY

Ken Hsu James F. Zucherman Arthur H. White

Recent advances in both fusion tech

niques and instrumentation havemarkedly facilitated the treatment ofspinaldisorders. Yet asignificantnumberof patients exist who continue to havepseudarthroses. Despite the surgical advances the essentials of a successful spinalfusion still appear to be the effective application of sound bone grafting principles. These principles, along with thetechniques, problems, and complicationsassociated with bone grafting, arc reviewed in this chapter.

The loss ofbone in the spine often presents serious difficulties not seen in other

areas. The most favorable replacementwould still be a bone graft that fills thedefect and becomes incorporated into thespine. However, the availability of appropriate bone to replace the loss is asignificant problem. Alternatives to bonegrafts, including a number of implantsused to stabilize the spine, are also surveyed.

Bone grafts have often played the roles

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of scaffolds, bridges, spacers, fillers ofdefects, and replacements of bone lost.Immobilization of multiple motion segments is frequently necessary in the spine;great demands arc made on bone grafts.In the lumbosacral spine, body weightand muscular forces impart loads equal tothree or four times body weight. It isnot surprising that the highest rate ofbone graft failure is seen in the lumbosacral spine. Hence, the following is adiscussion of technical problems, bio-mechanical and physiologic characteristics of bone grafts, and implants.

THE AUTOGRAFT

Autograft, or bone graft transplantedfrom one site to another in the same indi

vidual, is considered to be the most biologically suitable. Its advantages include:

1. Has superior osteogenic capacitya. Contributes cells capable of im

mediate bone formation

b. Allows for bone induction byrecipient bed where nonosseous

tissue is influenced to change itscellular function and become

osteogenic2. Lack of histoconipatibility differ

ences or ininiunologic problems3. Ease of incorporation4. No disease transmission '

The disadvantages include;1. Additional incision or wider ex

posure, prolonged operative time,and increased blood loss

2. Increased postoperative morbidity3. Sacrifice of normal structure and

weakening of donor bone4. Risks of significant complications5. Limitations in size, shape, quantity,

and qualityFor optimal results harvest autogenous

canccllous bone in the following manner.1. In thin strips

a. Not exceeding 5 mm in thick-

b. To provide maximal exposureof superficial cells

c. To allow rapid vascularization2. Graft wrapped in a gauze soaked in

patient's blooda. To avoid exposure to high in

tensity lightsb. Kept in temperature less than

42° C^'

c. Not stored in saline or antibioticsolution^-'"''

d. Without the use of chemicalsterilization'""

3. Transfer the graft to the recipientbed as soon as possible toa. Avoid exposure to air for more

than 30 minutes'"''

b. Protect the viability of the surface cells

4. Place the grafta. In wcll-vascularized bone bedb. In well-decorticated bone sur

face (cancellous site is superior)c. With healthy soft tissue cover-

age

5. Minimize surgical trauma because,

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Dotw Grnfts and JnipfaiUs in Spine Surgery 435

for example, high speed burringand inadequate irrigation retardhealing*'"'

6. Position the canccllous surfacea. On opposing cancellous surtaceb. On surrounding soft tissue with

good blood supplyc. So that total mass of graft is not

too thick to prevent nutrientdiffusion from recipient bed

7. Always avoida. Dead spaceb. Hematoma

c. Interposition ofnccrotic tissue8. To minimize risk, be aware of

a. Anatomyb. Potential complications

The iliac crest is the most versatile bonegraft reserve. It is relatively subcutaneousand easy to harvest in prone, supine, lateral, or other positions. It is expendable,and it has a large reserve of corticaland canccllous bone. In addition, it allows carpentering of different shapes and

Anterior Iliac Crest Grafts

Anterior iliaccrest bone grafts are usedfor anterior interbody fusion of the cervical, thoracic, or lumbosacral spine. Thesubcutaneous anterior superior iliac spineand iliac crest and easily palpable. Theiliac tubercle is the widest portion wherea large quantity ofcorticocancellous boneis found. (See Fig. 39-8.)

A skin incision is made parallel to or inline with the iliac crest. It is advantageousto center the incision over the iliac tubercle. The incision is carried down to thebone of the crest, and the muscles arcelevated subperiostcally to expose thewing of the ilium.

The tensor fascia latac, glutens medius,and glutens minimus originate from thelateralaspectof the ilium. They are innervated by the superior gluteal nerve. Theabdominal muscles are also attached tothe iliac crest and arc scgmentally inner-

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436 Spine Surgery: An AnihoIogY

vatcd. The incision over the crest is there

fore "internervous" and safe.

An appropriate osteotome or chiselmay be used to outline a cortical windowin the lateral iliac surface from which to

procure the bone graft. Longitudinalparallel cuts may be made (Fig. 39-1).Strips of cancellous bone may be removed with a curved gauge. Care mustbe taken not to violate the inner table of

the iliac wing where hernia is a significantpotential complication.

Bone graft may be obtained from theinner table of the iliac wing. However,there are risks of peritoneal perforationand significant bleeding with formationofhematoma in the retroperitoncal space.

It is important not to carry the incisionto or anterior to the anterior superior iliacspine. Injury to the lateral femoral cutaneous nerve or the inguinal ligamentmust be avoided. Detachment of the in

guinal ligament may result in inguinalhernia. If bicortical bone is taken too

close to the anterior superior iliac spine,fracture may occur. (See Fig. 39-6.)Avulsion of the anterior superior iliacspine may occur by the action of the attached muscles, such as the tensor fascialata or sartorius.

Bone may be removed in the form ofblock, dowel, strips, and by way ofcortical window or "trap door" (Figs. 39-1 to39-4). The iliac crest contour can be preserved by removing the bone deep to thecrest, or by temporarily detaching andrepositioning it later (Fig. 39-5). Theanterior superior iliac spine should be leftintact to maintain normal appearance.The region of the iliac spine should not beweakened by removing bone adjacent toit. Fracture and displacement of theinguinal ligament may result (Fig. 39-6).

The wound should be closed properly.The muscles and fascia must be sutured to

their original anatomic positions and thedefects closed; an effective drain shouldbe used.

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Fig. 39-1 Corticocanccllous bonegrnftisobtainedfromlateral iliac surface usinglongitudinalparallel cuts with an osteotome or chisel.

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Bone Grafts and Implants in Spine Surgery 437

Fig. 39-2 Hor.scshoc-sliapcd corticocanccllousbone graft is obtained froin iliac crest using ostco-tontcs positioned parallel to each other.

Fig. 39-3 Dowel-cutting instrument is used toobtain iliac graft with two tooled cancellous surfaces and cortical faces on three sides for anteriorspinal intcrbody fusion.

Fig. 39-4 Cortical "trap door" is used to gainaccess to iliac cancellous bone.

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438 Spine Surgery: An Anthofogy

Temporary detachmentof the iliac crest The iliac contour

Is preserved intact

The defect is filled

to avoid hernia

(Bonecement may be used)

Fig. 39-5 Large iliac graft isobtained with preservation of tlie iliac contour.

Anterior superior iliac spineL \ Inguinal ligament

Fig. 39-6 Illustration ofattachment of inguinal ligament to anterior superior iliac spine.Detachment ofingiiinal ligament may lead to inguinal hernia. Injury to lateral femoral cutaneous nerve should also be avoided.

Posterior Iliac Crest Grafts

The posterior iliac crest provides alargequantity of cortical cancelious bonegraft. The posterior superior iliac crest ispalpable under the skin dimple in thesuperior medial aspect of the gluteal region. The iliac crest curves cephalad andlaterally from the posterior superior iliacspine.

An oblique, curved or vertical incisionmay be made over the posterior iliaccrestor in line with it. The cluneal nerves cross

the iliac crest 7 to 12 cm anterolateral tothe posterior superior iliac spine (see discussion under Complications) and mustbe protected (sec Fig. 39-13).

A midline spine incision may be extended distally and the posterior iliaccrest approached laterally under the skinand subcutaneous fat. This avoids the use

of a second skin incision.

The incision is carried down to thebone of the crest, and the muscles arcelevated subperiosteally from the posterior lateral surface of the ilium. This approach docs not denervate the muscles.The gluteus maximus, medius, and minimus originate from the lateral surface ofthe ilium. The superior gluteal nerve innervates the gluteus medius and minimus, and the inferior gluteal nerve innervates the gluteus maximus. The para-spinal musculature innervated seg-mcntally originates from the iliac crest.

It is very important to remember thefollowing rules.

1. Stay on bone and work subperiosteally.

2. Avoid the sciatic notch and protectthe sciatic nerve.

3. Protect the superior gluteal vessels(see discussion under Complications) and protect the pelvic stabil-ity.

4. Avoid the sacroiliac joint.5. Protect the posterior sacroiliac liga

ments.

The removal of bone in the vicinity ofthe sciatic notch can weaken the thick

Bofic Crafts aitd Implants in Spine Surgery 439

bone that forms the notch. This can produce instability of the pelvis. It is important to stay cephalad to the sciatic notchand remove bone only from the false pelvis. For a landmark, an imaginary linedropped anteriorly from the posteriorsuperior iliac spine with the patientin theprone position can be used as the caudallimit of bone removal (see Fig. 39-15, Aand B). Care must be taken not to enterthe sacroiliacjoint, which may become asource of persistent pain and instabilitywhen injured.

A sharp surgical instrument (that is,an osteotonie or tip of Taylor retractor)may injure the sciatic nerve deep to thesciatic notch. Laceration of the superiorgluteal vessels is a significant danger inthis region. The vessels leave the pelvisvia the sciatic notch. A divided vessel caneasily retract into the pelvis and presentsavery alarming complication (sec discussion under Complications) (sec Fig. 39-

14)-Nutrient vessels supplying the ilium

found in the mid portion of the anteriorgluteal line may present troublesomebleeding and should be controlled withGelfoam, Surgicel, bone wax, or electro-coagulation.

A relatively painless bone graft donor sitefor lumbar spine fusion is possible byapplying the following technique. Aseparate incision over the iliac crest isnot made through the skin. The fasciathrough the wound of the lumbar surgery is grasped with Kochers clamps andpulled medially. The subcutaneous tissueis carefully elevated off the fascia laterallyand cautinlly until the fascia immediatelyabove the posterior iliac crest and posterior superior iliac spine isreached. ATaylor retractor is placed in the subcutaneoustissue lateral to the crest over the ilium

posteriorly. The periosteum is not dissected from the ilium except from thesuperior-most crest. The fascia is incisedover the crest, and an elevator is used toscrape the superior crest free of perios-

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440 Spine Surgery: An Anlliology

P.S.I.S.

Fig. 39-7 A gouge is used to remove posterior"roof of ilium aJid the cancellous bone betweencortical layers.

Iliac tubercle

A.S.I.S.

Fig. 39-8 Cancellous bone is removed from iliac tubercle, anterior or posterior iliac spineregion throughsmall cortical opening.

tcum to bare bone. Gouges arc used toremove the "roof" of the ilium and thecancellous bone between the cortical layers, leaving the cortices intact laterallyand medially with theirsoft tissue attachments (Fig. 39-7). This technique minimizes postoperative donor site pain andprevents uncomfortable scar tissue fromforming over the ilium, as when the lateral cortices are removed.

When limited quantity of cancellousbone is required, the following methodsmay be advantageous:

Cimcdage allows harvest of cancellousgraft with least morbidity through asmall round cortical window using asharp curette as shown in Fig. 39-8. Cancellous bone is most abundant in the pos

terior aspect ofthe iliac crest, followed bythe iliac tubercle and anterior superioriliac spine areas.

A "trap door" cut in theanterioror posterior outer table of the ilium and hingedon muscles can be opened to allow accessto cancellous bone. The trap door isclosed at the end. Postoperative pain appears to be less with this technique. Cosmetic deformity is minimal (sec Fig.39-4).

Wolfe andKawamoto'̂ ® reportedatechnique of obtaining full thickness bonegraft from the anterior ilium. Incision ismade through the iliac crest. The outerridges of the iliac crest are split obliquelywith the muscular and pcriosteal attachments remaining. All the iliac bone be-

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olfc and Kawamoto's technique ofthickness bone graft from the ante-sharp ostcotome is used to make

Its shown above and in Fig. 39-10.

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A large full-thickness bone graft canas shown, using Wolfe and Kawa-

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442 Spine Surgery: An Anthology

ncath tliis can then be removed. Theedges of the crest may be reapproxi-mated, thus minimizing cosmetic deformity, hernia, hcmatoina, and postoperative morbidity (Figs. 39-9 to 39-12).

COMPLICATIONS

Complications involving the iliac bonegraft donor site arc not uncommon. Although some of these complications maynot be serious, they add to the patient'sdiscomfort and prolong the convalescence. The complications that arc due tograft removal from the ilium include:

1. Major blood loss2. Hcmatoina.

3. Nerve injury (neuroma formation)

4. Severe pain (chronic pain)5. Hernia

6. Cosmetic deformity7. Fracture

8. Sacroiliac joint injury9. Pelvic instability

10. Hip subluxation11. Gait disturbance

12. Peritoneal injury13. Uretcral injury14. Hcterotopic bone formation15. Infection

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Fig. 39-12 Wolfe and Kawamoto's technique ofrcapproxiinating the two fragments of die crest,using wires or sutures. Figure eight wire or suturemay be passed through the bone witli an awl andfixed to adjacent bone.

Cockin^'' reviewed 118 cases of iliac

crest bone graft procedures and foundmajor complications in 3.4% and minorcomplaints in 6%. There were two casesof meralgia parcsthctica, one of hernia,and one ofhip subluxation after extensiveremoval of the iliac crest. The minor

complaints included wound pain, hyper-sensitivity, and buttock anesthesia.

Nerve Injuries

Possible nerve injuries include the following.

1. Lateral femoral cutaneous

nervc2'^-"«

2. lliohypogastric (lateral cutaneousbranch) nerve

3. Superior cluneal nerve (cutaneousbranches of dorsal rami LI, L2,L3)2r>.3r,

4. Middle cluneal nerve (cutaneousbranches of dorsal rami SI, S2, S3)

5. Sciatic nerve

6. Ilio-inguinal nerve'®7. Femoral nerve

8. Superior glutcal nerveSuperior cluneal nerves are lateral

branches of the posterior primary division ofthe upper three lumbar nerves thatrun posteriorly through the lumbosacralfascia at the lateral origin ofsacrospinatus


Superior cluneal nervesdorsal rami LI

2 L

Posterior superioriliac spine'

MiMiddle cluneal nerves LjJIcutaneous branches

dorsal ramI2

liiohypograstrlc nervelateral cutaneous branch

Lateral cutaneous branch

\ VV ofsubcostal nerve (T12)

Anterior superioriliac spine

Gluteus maximus- ♦ '^llio-tibial tract

Fig. 39-13 Illustration ofthe nerves that may be injured during the procedure to remove bonegraft from the iliac crest.

muscle. They crossover the dorsalaspectof the posterior iliac crest and providesensation to the skin of the buttocks.Theyarc found? to 12 cmantcrolateral tothe posterior superior iliac spine in theadult. 'When incision is made across orparallel to the posterior iliac crest, thecluneal nerves may be injured (Fig. 39-13).

Painful neuritis of the buttocks hasbeen reported.That these nervesare acause of disability can be demonstratedby relief of symptoms after they havebeen infiltrated with local anesthetics.Permanent relief can be obtained by resection of the nerves with the transectedends allowed to retract into the soft tis-

Thc sciatic nerve may be injured whenthe dissection is extended down to thesciatic notch. A surgical instrument suchas an osteotomc may be passed deep tothe sciatic notch to cause this injury. Thebony rim of the notch should bepalpatedbefore the dissection is carried to thisarea. An imaginary plumb line droppedfrom the posterior superior iliac spinewith the patient in the prone position willpass tlirough the bony rim of the sciaticnotch. This serious complication can beavoided if you stay cephalad to this line

A

(see Figs. 39-14 and 39-15).The ilioinguinal nerve may be injured

when the abdominal wall is retractedmedially from the anterior iliac crest. Thenerve may be compressed beneath theretractor on the inner part of the wall ofthe ilium. It occurs when the inner cortexof the anterior ilium is exposed for removal of bone grafts. Ilioinguinal neurologic injury is characterized by pain radiating from the iliaca toward the inguinaland genital areas. This complication iswell discussed by Smith and associates.

The iliohypograstric nerve(lateal cutaneous branch, LI ventral rami) is foundover the midlateral aspect os the iliaccrest. It shouldbe protected when working in this region (sec Fig. 39-13).

Vascular Injuries

Vascular injuries may include the superior gluteal artery (and vein),""'" thedeep circumflex iliac artery, the iliolum-bar artery, and the fourth lumbar artery.

The superior gluteal artery is a branchof the internal iliac artery that curvesaround the rim of the sciatic notch as itleaves the pelvis. It may beinjured whendissection is carried close to the sciaticnotch. An osteotomcor thesharp point ofa Taylor retractor may enter the notch

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Poslerior superioriliac spine

Superior giuteal artery

Piriformis

Sciatic nerve

Fig. 39-14 Illustration of the superior glutca! artery, curving around the rim of the sciaticnotch as it leaves the pelvis.

and pose similar danger to the artery.This complication can become alarming,since the divided vessel easily retracts intothe pelvis (Fig. 39-14).

If the superior gliiteal vessel is lacerated, it can be compressed locally andexposed for ligation or clipping. A fingermay be used to apply direct pressure tothe vessel against the bone. Kahir^' discussed the use of a Raney-modified Ker-rison rongeur to remove the upper margin of the sciatic notch to expose thebleeding vessel. If the bleeding vessel isstill not accessible, the patient may bepositioned for a retroperitoneal or trans-peritoneal exposure of the vessel. Arterialocclusion by cmbolization or using aFogerty catheter is another option.

Injury to the superior giuteal vesselscan be prevented if the surgeon is wellaware of the anatomy in this region. Thebony origin of the glutens maximus orthe roughened area anterior to the posterior superior iliac spine is a good landmark and can be used as the caudal limit

of bone removal (Fig. 39-15, A and B).An imaginary plumb line dropped fromthe posterior superior iliac spine with thepatient in the prone position will pass

through the bony rim of the sciaticnotch.It is important to stay cephalad to thisline.

Escalas and DeWald''" reported a caseof combined traumatic superior giutealarteriovenous fistula and ureteral injurycomplicating removal of bone graft fromthe posterior ilium. The tip of a Taylorretractor accidentally dislodged andpenetrated into the sciatic notch to causethis unusual injury.

The deep circuinjlcx iliac artery, theiliolwnbarartery, or thefourth lumbararterymay cause troublesome bleeding whenworking on the inner table of the ilium.Occasionally, peritoneal perforation accompanies the arterial injury. The anatomic position of the arteries are illustrated in Figs. 39-16 and 39-17. It isvery important to stay subperiosteallyand carefully elevate the abdominal wallmuscles off the crest and the iliacus mus

cles off the inner table of the ilium (Fig.39-18).

A hernia through the iliac bone graftdonor site may occur after the removal offull thickness bone from that site. It mayappear as an iliacswelling, sometimes associated with pain or symptoms of bow-

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Posterior superior Iliac spi

Superior gluteal artery

Fig. 39-15 A, The bony origin ofthe glutcus maxtinus or the roughened area anterior to theposterior superior iliac crest is agood landmark and can be used as the caudal limit of boneremoval. An imaginary plumb line dropped from the P.S.I.S. with tiie patient in the proneposition will pass through the bony rim of the sciatic notch. The superior gluteal artery isadjacent to the bony rim. B, Alarge amount ofbone graft can be removed safely ifthe surgeonstays ccphalad to the RS.l.S., the sciatic notch and the imaginary line joining them.

Fourth lumar artery

lliolumbar artery

iDeep circumflex iliac artery

Fig. 39-16 Illustration of theanatomic positionsof the arteries that may cause troublesome bleeding when working onthe inner table ofthe ilium.

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Fig. 39-21 The iliac wall is reconstructed withbone cement.

cl obstruction.Strangulatedhernia and valvulac arc very rare occurrences.^' Syniptoins were reported tohave occurred from 24 days^' to 15years'""' after the formation of the iliacdefect.

Treatment requires the reduction ofthe hernia and repairing the defect by:

1. Using soft tissue'-^'advancement, imbrication, flaps, or fascialflaps

2. Using a prosthesis"" (tantalum orMarlex mesh)

3. Using methylmcthacrylatc cementto reconstruct the iliac wall"" (Figs.39-19 to 39-21)

4. The Bosworth technique:''" removing the remaining wings of theilium on cither side of the defect

followed by layered soft tissue clo-

Pclvic Instability

Removal of a large quantity of bonegraft from the posterior ilium may disrupt the mechanical keystone effect of thesacroiliac joint and the posterior sacro-iliac ligament, causing instability. Licht-

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Boiic Grafts and Iniplaiits in Spine Surgery 447

bloif'" first reported such complicationsalter a bone grafting procedure in whichthe posterior sacroiliac ligaments werepostulated to be interrupted. The ensuinginstability transferred the stress forces tothe pelvic ring, causing fractures of thesuperior and inferior pubic rami. Coventry and Tapper^" reported six cases ofpelvic instability following removal ofbone graft from the ilium. The patientswith such instability often developedsymptoms indistinguishable from otherspinal disorders. History of clicking orthudding, as well as pain in the thigh andgluteal region, is characteristic.

Sacroiliac stability is maintained byformation of the sacrum as a keystonewith interlocking eminences and depressions, plus ligamentous support mostlyin the posterior and superior aspect.'"Multiparous women with lax ligamentsand anatomic variations in the sacroiliacjoints are more prone to develop suchpelvic instability. Radiologic examination of the entire pelvic ring is important.Changes in the sacroiliac joint, the pubicrami, and the symphysis pubis should belooked for.

The Tibia

Thc/i7;/V7 provides strong full thicknesscortical graft and is occasionally used inspine fusion. The subcutaneous antero-medial aspect of the tibia is a convenientdonor site. The periosteum should be leftintact and sutured over the defect. The

condyles also supply cancellous bone.However, there are significant risks tothe use of the tibia as a donor site. Bio-

mechanically, it is changed from a closedsection to an open one when bone graft isobtained from the tibia. It is markedlyweakened and much less able to resisttorsional and bending loads.

Frankel and Burstein''^ discussed the

effect of cortical graft removal from thetibia. They described the torque andangular deformation to failure of the tibiato be reduced to 30% of normal and

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energy absorption capacity to 10% ofnormal. Even when the corners of the

cutout arc rounded, open section overshadows any reduction in stress concentration gained.

Fatigue fractures arc relatively high,and the tibia should be cast immobilized

from 6 to 12 months after bone graft isobtained." Thu.s the disadvantages ofautogenous tibial graft far outweigh thebenefits.

The Fibula

Although the upper two thirds of thefibula may be removed as bone graft, themiddle one tliird provides the best cyl-indric cortical bone graft. The fibula graftisstrongest in resisting comprcssivc loading and can be depended on for longerperiods of structural support in interbodyfusion. For large defects in the vertebralbodies, fibula struts may be used toachieve stability. Because of the smallamount ofcanccllous bone in the fibula,iliac cancellous graft should be supplemented to enhance osteogcnesis.

Peroneal nerve injuries may occurwhen obtaining the graft from the proximal one third of the fibula. Valgus deformity of the ankle is a serious risk whenthe lower one third is violated. Significant donor site pain and compartmentsyndrome have also been reported.

Ribs have been used for thoracic spinefusion. However, their modest cortexand porous cancellous bone are rarely appropriate for lumbar spine fusion.

Free Vascularizcd Bone Grafts

Free vascularizcd bone grafts may beused to circumvent the disadvantage oflarge cortical grafts, most of which become necrotic.'®'®" Recent progress in mi-crosurgical techniques is making this possible.'"'"® Continuing circulation and increased viability of the bone grafts facilitate the problem to that of fracture healing. Vascularized grafts are less depen

dent on the recipientbed forsurvival, andtheir use is advantageous in poorly vascularizcd bed after previous .surgery,trauma, infection or irradiation. Thefibula, rib, and anterior or posterior iliummay be used. However, their applicationis usually limited by the small size andneed for time-consuming highly specialized microvascular techniques. In specialcircumstances the use of free vascularized

bone graft may be advantageous in spinefusion.

Dupuis and coworkers" used a freevascularizcd fibular graft in a case of progressive congenital kyphosis with success, following the work of O'Brien andOstrup. In a similar situation, an avascu-lar strut graft becomes weaker to thepoint of mechanical failure as it is replaced by creeping substitution, whichmay take two or more years to complete.

Muscle-pedicle bone grafting procedures were reported by Hartman and associates for failed lumbosacral spinal fusion.''® An iliac crest autograft with anintact quadratus lumborum muscle pedicle was used in this case.

ALLOGRAFTS

Allografts arc the most frequently usedalternatives to autografts in spine surgery.They are bones transplanted from oneindividual to another and are used to cir

cumvent the problems encountered withautografts.

Allografts are readily available andcome in a wide variety of shapes andsizes. They can provide immediate support and minimize the use of stabilizationhardware or braces. Bone allografts canreplace missing structures and becomeincorporated into the spine. They provide biologic scaffolding that isgraduallyreplaced with the patient's own bone.

Major problems lead to decreased effectiveness of allografts.Immu-nologic rejection of implanted graft,®''"®delayed union, nonunion, and fracture of

the graft have not been uncommon. Incorporation of allografts by the host isslower. Vascular penetration is slowerand less dense. There is less perivascularnew bone formation when compared toautografts. Transmission of disease fromallografts is also a serious concern.

The major weakness of the allograft isthat it is dead and cannot contribute directly to osteogenesis, as do fresh autografts. Burwell'"' found a way around thisproblem by combining the osteogenicpotential of autogenous marrow with allografts. The use of autogenous marrowto provide superior osteogenic capabilityin allografts and xenografts, as well asautografts, is finding greater clinical application (see further discussion on xeno-graft and synthetic implants). The use ofbank bone is very advantageous ifstorageproblems, ininuinologic reactions, andinfection could be eliminated.

The allograft must be aseptically obtained soon after death or properly sterilized and processed early to:

1. Minimize its antigenicity2. Prevent degradation by protcolytic

enzymes

3. Maintain the mechanical structure4. Preserve the osteogenic induction

property

Freezing and Frecze-Drying

Freezing and freeze-drying are themost widely used preservation methodsthat allow storageof bone in a biologically useful (but nonviable) state.*

Freezing of allografts is carried out assoon as possible after procurement. Currently, the length ofsafestorage for boneis not known. However, ba.scd on theknowledge of autolysis retardation bycold, lower temperatures arc expected toextend the "shelf life" of allografts.'*^ At—15® C to —30® C, using a home type ofmechanical freezer, long-term storage of

^References 11, 43, 61, 62, 70. 71.


bone is difficult. This form of freezing isnot advisable because ice crystals growrapidly in this temperature range andmechanically destroy the tissue viability.^' Freezing at —76® C is achieved indry ice. At -60® C to -90® C usinga laboratory type of mechanical deepfreezer and at —150® C or colder using arefrigerator with cryogenic gases, moreeffective preservation of boneis possible.At temperature near —70® C, ice crystalformation is slower." Bones frozen to—70® C have beenstored for several yearsand successfully applied clinically.'*®Freezing in a cryoprotcctive agent such asglycerol at controlled cooling velocitymay be a more effective option.

Freeze-drying is a process in which thebone is first frozen to —70® C and thensublimated in liigh vacuum. The bone isfreeze-dried until the water content is reduced to 5% or less. The frccze-dricdbone graft then can be shipped andstoredconveniently at room temperature indefinitely in a vacuum container. Sincefreeze-dried bone is very brittle, it mustbe reconstituted by immersing in normalsaline before use.'® The reconstitutiontime depends on the size andshapeof thegraft. Chips of bone may not require anyrehydration, whereas larger cortical bonemay require up to 24 hours for reconstitution."*®

As for their clinical application in spinal fusion, Malinin and Brown" reportedthe union rate of freeze-dried allograftsunder compression load (interbody orstrut grafts) not to be delayed by lowgrade imniunologic response. This is incontrast to a high incidence of resorptionwith cortical graft placed under tensionposteriorly in the spine.

Bioniechanical Properties

Biomechanical properties of bonegrafts may be changed by the techniquesused for preservation, although Sedlin'sstudy showed that freezing and thawing

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450 Spine Surgery: An Anthohi^y

do not significantly change the mechanical properties of bone.'-"'

Briglit and Burstein,^ as well asTriantafyllou and coworkers,'"® studiedthe biomechanical properties of freezc-dried and irradiated bones. Komcnder""

found that freezing to —78° C docs notalter the mechanical properties of bone.Pelker and associates®" also concluded

that freezing allograft bone to temperature as low as that of liquid nitrogen(—196° C) docs not significantly alter thebiomechanical properties. They showedthat freeze-drying docs diminish the tor-sional and bending strength but not thestrength in compression. The data ofPelker and coworkers indicate that frozen

bones arc better suited than frceze-dricd

bones when they are subjected to tor-sional loads.®"'®"

Both frozen and frceze-dricd bones are

acceptable when comprcssive forces arethe primary concern. It must be remembered that the initial biomechanical properties of the bone graft will change withrcsorption, incorporation, and remodeling by the host. Surgical technique, internal fixation, and postoperative management must therefore be planned accordingly.

Radiation, Heat, and

Chemical Treatment of Allograftsand Xenografts

Although most of the ascptically procured cadaver bones do not require sterilization procedures, allografts or xenografts have been sterilized by physicalmeans such as high energy radiation, andheat (boiling, autoclaving); and by chemicals such as merthiolate (Thimcrosol),cthylene dioxide, Propiolactone, or usingantibiotic solutions. Some of these steril

ization methods were used in the past andare discussed for historical interest only.

Radiation of at least 2 mcgarads is required to kill bacteria; 4 megarads inactivates some viruses.^' The same dose of

^ n'

radiation (2 to 4 mcgarads) needed forsterilization or to destroy antigens, alsosignificantly impairs the inductive repaircapacity of the bone graft.Increased solubility of collagen and glyco-saniinoglycan, destruction of bone matrix fibrillar network'^''® and discolora

tion^' of irradiated bone have been re

ported. In cobalt-60 irradiated bone Os-trowski®" reported free radicals of unusual stability to be present, althoughtheir effect on the host tissue is unclear."

The effect of radiation on the biome

chanical properties of bone is not welldefined at this time. The effect seems to

be minimal with low level radiation.

Radiation doses exceeding 3 megaradsarc known to destroy bone matrix fibrillar network.'^''" There appears to be significant drop of breaking strength ofbone with more than 3 megarads. Thiseffect is magnified when radiation iscombined with freeze-drying. Ko-mcnder noted 6 megarads of radiationreduced the strength in bending, compression, and torsion."" Irradiation offrecze-dried bone with only 3 megaradsmarkedly diminished the bendingstrength, but not the strength in compression or torsion.

Boiled bones have been used for graftmaterial since the early part of this century.''" Although some good clinical results have been reported,""'"^'' boiled allografts and xenografts have generally produced undesirable consequences. Boilingdestroys all inductive capacity.'^'' Heatmay be intended to destroy transplantation antigen, but it only denatures theantigenic proteins into another unacceptable material.

Autoclaving of contaminated bonemay be tempting to use in the operatingroom, but this produces haversian canalcoagulation and denaturation of boneprotein,which severely retard hostincorporation.

Chemical processing ofbone graft may

present significant problems such as potential carcinogencsis and difficulty ofpenetration into bone. Propiolac-tone (1% solution) has been found to bemore bacteriocidal than cthylenedioxide,which is more difficult to use.'"

Merthiolate-treated grafts have in general produced poor results; 30% of thegrafts failed in the study by Reynolds andcoworkers.'-'̂ There appears to be threetimes as many failures as with autografts.Reduced callus formation and osteogene-sis have been noted. When the graft fractured, there was minimal healing. Whenwashed before use, no significant hostsensitivity to Merthiolate was noted inMerthiolate-treated bone graft.''

Bcnzalkonium chloride completely destroys osteogenic inductive capacity ofbone, according to Urist and associates.'" Antibiotic solutions do notpenetrate completely into bone. Theirgermicidal effect inbone graft is variable.In general, antibiotics appear to inhibitthe osteogenic inductive , capacity ofbone.'''"

Bone Morphogenetic Protein

Urist states that allograft bone must beremoved from the donor within 4 to 8hours after death or within the minimalbiodegradable time.'" Radiation sterilization with more than two megarads,heating over 60° C, exposure to chemicals such as hydrogen peroxide, beta-propriolactone, bcnzalkonium chloride,cryolysis, immediate freeze-drying, andprolonged storage at 0° to 30° must beavoided to preserve the inductive properties of the bone. Urist and coworkershave extensively studied osteogenic induction and discovered a bone morphogenetic protein (BMP) that is capable ofinducing the differentiation of host peri-vascular mesenchymalcells into cartilageand bone."'' They have separated BMPfrom demineralized cortical bone andosteosarcomas of man and mouse. BMP

;?r-

Botw Grafts and Implants in Spine Surgery 451

is characterized as glycoprotcin(s). itsclinical application now is being evaluated in the form of an injectable substance linked to different delivery systems."''

AAA bone is a chcmosterilized, auto-digested, antigen-extracted allograft developed and clinically tested by Urist andcoworkers. It is an allogeneic bone ofhigh osteogenetic property and low im-munogenicity prepared in five basicsteps. Urist believes that BMP is also preserved by these measures.

1. Lipids and cell membrane lipopro-teins are extracted using chloro-form-mcthanol.

2. Endogenous intra- and extracellulartransplantation antigens are removed by neutral phosphate bufferautodigestion in the presence ofsulfhydral group enzyme inhibitorsto preserve BMP.

3. Acid-soluble proteins are extractedand matrix demineralized by 0.6 Nhydrochloric acid.

4. The bone is frceze-dried. Residualproteins including BMP are preserved.

5. The processed AAA bone is storedin sterile double plastic envelopeand outer vacuum-sealed glass container.

Urist and Dawson"" reported 40 inter-transverse process fusions in 36 cases ofdegenerative joint and disc disease including spinal stenosis and spondylolis-thcsis, aswell as4 cases of thorocolumbarfracture dislocation. A composite ofAAA cortical bone strips and local auto-logous bone was used in all cases. Therewere over 80% excellent and good resultswith a nonunion rate of 12%.

XENOGRAFTS

Xenografts, or bones transplantedfrom other species, have been used inspine surgery.The application ofivory,'"'"" animal horns,^"' corals,and

vf.rntK'w.

452 Spine Surgery: An Anlhology

other exotic materials has been explored. fusions that failed. Biopsies of the KielAnimal horns and ivory arc very resistant bone implants showed invasion by fi-to incorporation into the host bone.®'' brous tissue. There was no ossificationFresh xenograft bones have been shownto be unacceptable. Invariably, they produce inflammation, fever, sequestration, be invaded by host new bone whenresorption, or other manifestations of rejection.'^ Fibrous envelopment occurs asover a metal plate. Even when fusiontakes place, sequestration of the xenograft is observed. Urist believes that scaffolding with good bone conductionxenografts should not be used in pa- property.Salama®'' and Salama andtients.'" coworkers"®'"® reported good results

Bovine bones have been relatively using autogenous bone marrow-Kielpopular because they incorporate and bone composite grafts in patients. Theremodel with less difficulty.®'' Different red marrow can be easily aspirated from

the patient's own iliac crest.

SYNTHETIC IMPLANTS

Synthetic implants can be prepared tofit any size or shape, but they have beentraditionally considered to be subject to

calcified calf and sheep bone) cval- wear and not biologically incorporateduated experimentally and clinically,but found unsatisfactory

Deproteinized xenografts, including "ospurim," "anorganic bone," "Oswestry

types ofpreserved bovine bone have beentried since the nineteenth century.

1. Frozen calf bone

2. Frceze-dried calf bone (Boplant)®'and

3. Decalcifiedox bone'''' (as well as de-

been tried.

able. Experimental studies showed that itis very weakly antigcnic and does notpossess active bone-inducing capacity.®®

have been reported clinically.In spinal fusion, Jackson'̂ "' noted that

Kiel bone implantbecame surroundedbyautogenous bone with time. For largerdefects he recommended the use of auto-

and no incorporation into the surrounding bone. Such deproteinized bone could

I

placed in excellent vascular bed with potentially osteogenic cells. When impregnated with autogenous bone marrowcells, it may prove to be an excellent

into the host bone.'^ A number of im

plants fashioned from metals have beentriedas replacement for bonein thespinal

tanium vertebral replacement: "TotalVertebral Body and Pedicle Replace-nient"'"''''"^').

bone from freshly killed calf, sterilized * Jeither by cthylenc dioxide or by gamma being replaced may be covered byradiation, has been commercially avail- ground autologous bone grafts of small

particle size. In animal experiments ingrowth of bone occurred over the totalsurface area of fiber metal implants and

Since its introduction in 1957, Kiel bone bone penetrated deep into the compo-has been used in almost every possible site.^bone graft site, and varying success rates

Metal scajfolds with the shape of bone

Titaiiiitni mesh implants have been din-ically applied by Lcong,®^- '̂' and cowork-crs for anterior spinal fusion after discec-tomy in the lumbar spine. This porousimplantallows ingrowth of bone andappears to obviate the use of bone graft. It

genous and Kiel bone composite. Mc-Murray'® presented clinical, radiologic,and histologic data on the fate of Kiel the slow ingrowth ofbone and long-termbone implants in four anterior spinal stability.

acts as a spacer and can provide immediate stability, while allowing time for

blocl

^row

12-y.paiieiinplundi

5-ye,were

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Experimentally, porous titanium meshblocks with a 50'/o void allows rapid ingrowth of bone in canine long bone. A12-ycar follow up was possible in twopatients who are asymptomatic, and theimplants have remained unchanged andLindisplaced; 10 patients had more than5-year follow up. Of these, seven patientswere asymptomatic, two had more than70% symptomatic relief, and one retained avery stifTback. Radiologic analysis showed that disc height was maintained at 5 years with no movement between the adjacent vertebral bodies, oftenwith bony overgrowth anterior to theimplant.

Nonmctallic Synthetic Implants

There is a growing number of othersynthetic implants being used as bonesubstitutes. According to Osborn andNemcsley"''̂ the chemical nature of theimplant determines the biodynamics andreaction of the recipient bed in the interaction with living bone. They consideredthe following materials.

1. Bone cement and stainless steel asbiotolawU, resulting in distance osteo-genesis with a fibrous layer separating the implant from bone

2. Alumina and carbon materials asbioincrl, resulting in contact ostco-genesis

3. Glass ceramic, calcium phosphateceramics and hydroxyapatite ceramicsasbioactiue, resulting in bonding osteogenesis

Bioinert porous ceramics of aluminawore noted by Bcnum and associates tobe bound to bone by the ingrowth ofbone 3 to 4 mm thick in regions exposedto compressive forces.

Thus far, evidence strongly suggeststhat porous calcium phosphate ceramicsare the most biocompatible syntheticbone substitute with the ability to become chemically bonded by living boneand with a chemical composition devoid

of toxicologic liabilities."*' They areshown to be superior to biodegradablepolymers, such as polylactic acid andpolyclycolic acid, wliich have been considered as bone substitutes.^"'"'' The implants may be in dense form or porous.The minimum pore size for ingrowth ofbone is shown to be 100 /Am."** Coralsprovide such porous structures.'"'"^

Holmes and coworkers'''' performedhistologic and biomechanical studies indogs using hydroxyapatite convertedfrom sea coral calcite as bone substitute.The material was incorporated in boneand became almost as strong as the nativebone. They also reported encouragingclinical application with fractures in 18patients.

Another material, Replam Hydroxy-apatite-Poritcs (RH AP). is aceramic withthree-dimensional interconnected porousmaterial of calcium hydroxyapatite fromthe exoskeleton ofporitcs (coral). It maybe carved by the surgeon before implanting. RHAP was approved for evaluationin spinal fusion in several centers underMooney and associates. (See Chapter 41.)

Bioactive and biodegradable porousceramics ofhydroxyapatite or tricalciumphosphate have been studied. Jarcho'stated that they are usually well toleratedand become chemically bonded to boneby natural bone-cementing mechanisms.

Porous hydroxyapatite ceramics havebeen used in dog experiments for thespinc^" and other skeletal defects. Porousceramics and autologous marrow composites were studied by Nade and associates." Porous alumina, calcium alum-inate, calcium hydroxyapatite, and tricalcium phosphate were placed with bonemarrow into intermuscular sites. Bonewas found to adhere to the ceramics andto penetrate the interior if the pore sizewere greater than 100 ^m. The marrowcells were shownto play asignificant partin new bone formation into the framework. Nade and coworkers believe that

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the appropriate liistocoiupatible biodegradable ceramic material would act as ascaffold by virtue of its porosity for retention of bone marrow cells, and provide mechanical strength, while bone ingrowth is progressing. This type of bonesubstitute would also allow a wide selection of sizes and shapes in sterileform.

Porous biodegradable ceramic andBMP composites were evaluated byUrist and coworkers."'^ They reportedthat an aggregate of B-tricalcium phosphate and bone morphogenetic protein(TCP/BMP) induced the differentiationof cartilage in 8 days and in lamella bonein 21 days. The yield of new bone wasmore than 12 times greater from theTCP/BMP than from the BMP alone, it

is possible that a porous ceramic acts as aslow-release delivery system to distributeBMP more favorably and to potentiate itsactivity.

Calcium phosphate-coated metallicimplants showed superior bone-bondingcharacteristics according to Ducheyncand associates.^" Such implants may solvethe problem of weak mechanical strengthof ceramics, particularly the porousones.Ceramic implants by themselves areprobably unsuitable for restoration thatwould have to withstand significant impact, or torsional or bending stresses,''" asin the spinal column.

Calcium phosphate-containing bonecements are also being developed.''"'"Recently, calcium hydroxyapatite inpowder form was used as an expander ofa patient's own cancellous bone graft. Ithas been used in spine fusion, especiallyin children, when there is insufficientautologous bone graft available.""®

METHYLMETHACRYLATE

CEMENT

Knight"" was the first to report the useofacrylic cement to fix the cervical spineswith chronic fracture dislocation, at-

lanto-axial subluxation, and cervicalspondylosis. He also stabilized the lumbar spine using the cement in one patientwith disc disease. Scoville and cowork-

ers"'̂ reported the use of acrylicplastic forvertebral replacement or fixation in met-astatic tumor destruction of the spine.

In recent years the clinical use ofmethylmethacrylate cement for spine stabilization has become more popular.Harrington''" documented the use ofmethylmethacrylate for vertebral bodyreplacement and anterior stabilization ofthe spine with metastatic tumor. His series included 14 patients treated by anterior decompression and stabilizationusing metal and the bone cement.

The strength of methylmethacrylate isabout one half that of bone.'" Attemptshave been made to strengthen the cementby adding fibers," but clinical data arcstill unavailable. After polymerization,methylmethacrylate becomes a rigid andbrittle solid that can withstand significantcompression. However, it fails undertension or shear forces. It is reasonable to

use it in replacement of vertebral bodywhere compression is the predominantforce present. It is important to remember that when used alone the outer part ofthe cement mass is still subject to tensionwhen bending and will fail with time in aclinical setting. The primary indicationfor application of methylmethacrylate inspinal stabilization is in patients with malignant disease and limited life expectancy.""'̂ " It shouldnot beexpected to provide long-term support of the spine."'

In the spinal column, methylmethacrylate cement should be used with secure metal fixation such as the Dunn,""-""'""Harrington, Luquc, Steffee,orother instrumentation. It may be used asreinforcement for screws and hooks in

cancellous bone. The cement docs en

hance fixation of implants by increasingthe contact area, especially in osteo-porotic bone.

T'.:

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Bone Grafts and Implants in Spine Sin^i^cry 455

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>« r i. -

r

" n 1^.

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jiiniDmiiii'wiiitim

1


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UVIIL > '

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^ i i t. -miv f-Ki"-Of

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