Biomechanical Evaluation of the Origin of the Long Headof the Biceps Tendon

Jack H. Healey, M.D., Shane Barton, B.S., Phillip Noble, Ph.D., Harold W. Kohl III, Ph.D.,and Omer A. Ilahi, M.D.

Purpose: Lesions of the superior glenoid labrum extending anterior and posterior (SLAP) haverecently been recognized as important sources of shoulder pain and dysfunction. Among the 4described types of SLAP lesions, the type II SLAP involves detachment of the superior labrum fromthe bony glenoid and destabilization of the origin of the long head of the biceps tendon (LHBT). Thepurpose of this cadaveric biomechanical study was to evaluate the relative contribution regardinglinear stiffness and displacement under load of the 2 origins of the LHBT: the superior glenoidlabrum and the supraglenoid tubercle (the biceps anchor).Type of Study: Cadaveric biomechanicalstudy.Methods: Seven pairs of fresh-frozen cadaveric shoulders were dissected free of all soft tissueexcept for the glenoid labrum and LHBT. Tension from 0 to 55 N was applied to the LHBT whilekeeping the tendon perpendicular to the face of the glenoid. Each specimen was tested for linearstiffness and biceps tendon displacement in the intact state, after releasing 1 of the LHBT origins, andafter releasing the remaining origin.Results: The average stiffness of the LHBT origin was 103N/mm. Sectioning the anchor alone resulted in a 52% reduction in linear stiffness, whereas onlydetaching the superior glenoid labrum from the 10 o’clock to the 2 o’clock position resulted in a 15%reduction in linear stiffness. Maximum displacement of the biceps tendon origin in the intact state atthe 55 N load averaged 0.99 mm. With a minimum load applied, displacement changed less than 1mm unless both origins were released.Conclusions:The results indicate that the biceps anchor is theprimary restraint of the LHBT and that the superior labrum is a secondary restraint in regard to linearstiffness. However, disruption of both restraints is required to produce the laxity typically seen in atype II SLAP lesion.Key Words: Biceps—SLAP—Labrum—Biomechanics—Origin.

Lesions specific to the superior labrum and longhead of the biceps tendon (LHBT) origin were

not well recognized until the introduction of shoulderarthroscopy. Andrews et al.1 first reported arthro-scopic findings of anterosuperior labral tears involvingthe LHBT origin in baseball pitchers and other throw-ing athletes. Snyder et al.2 later described similarlesions and coined the term SLAP lesion for injury to

the superior labrum extending anteriorly and posteri-orly. They categorized SLAP lesions into 4 typesdepending on the morphology of the labral tear andthe extent of involvement of the LHBT. The type IIlesion is defined by detachment of the superior labrumand biceps anchor with resultant LHBT origin insta-bility. Arthroscopic diagnosis of type II SLAP lesionsis challenging because of the normal anatomic varia-tions of the soft tissues in this area. Examples ofnormal variations of the superior portion of the gle-noid labrum include the sublabral foramen, the me-niscoid superior labrum, and the Buford complex.Each of these normal variations can be misinterpretedas pathologic detachments of the labrum.

The LHBT has been shown to be an importantstabilizer of the shoulder and loses much of its effectafter sectioning of its origin.3-5 However, the biome-

From the Baylor Sports Medicine Institute, Department of Or-thopedic Surgery, Baylor College of Medicine, Houston, Texas,U.S.A.

Address correspondence and reprint requests to Omer A. Ilahi,M.D., Baylor Sports Medicine Institute, 6560 Fannin, Suite 400,Houston, TX 77030, U.S.A. E-mail: oilahi@bcm.tmc.edu

© 2001 by the Arthroscopy Association of North America0749-8063/01/1704-2476$35.00/0doi:10.1053/jars.2001.21244

378 Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 17, No 4 (April), 2001: pp 378–382

chanics of the individual components of the LHBTorigin, the superior glenoid labrum and the supragle-noid tubercle, has not been reported. The purpose ofthis study was to evaluate the relative contributions ofthe 2 origins of the LHBT in terms of linear stiffnessand displacement under load.

METHODS

Specimens

Seven matched pairs of fresh-frozen cadavericshoulders were selected, 5 from males and 2 fromfemales. The average age at time of death was 57years and ranged from 42 to 72 years. The specimenswere thawed and dissected free of the humerus and allsoft tissues with preservation of only the scapula, thelabrum, and the entire length of the LHBT. No ana-tomic variations were found in any of the specimen.

Each scapula body was potted in cement with theglenoid facing superiorly, and the plane of the glenoidpositioned parallel to the ground. The glenoid wasmarked in clockwise fashion using the superior apexof the bony glenoid as the 12 o’clock position. The 10and 2 o’clock positions were specifically marked.Acrylic cement was applied to the bony surface of thescapular neck medial to the glenoid to create a flatsurface for the attachment of infrared markers. Thebiceps tendon was secured by wrapping it twicearound a1⁄2-inch stainless steel rod and then sutured toitself with No. 2 braided, nonabsorbable suture. Eachtendon was sutured to allow 5 cm of free tendon asmeasured from the glenoid to the steel rod. Neutralrotation of the biceps tendon was maintained.

Monitoring

A computer-integrated infrared photometric system(Qualisys, Glastonbury, CT) was used to monitor thedisplacement of the biceps tendon during testing. Nineinfrared markers, 2.5 mm in diameter, were attachedwith cyanoacrylate adhesive to predetermined loca-tions on the specimen. Three markers were placed at1-cm intervals in line on the biceps tendon, 3 along theperiphery of the labrum at 1-cm intervals centered atthe biceps origin, and 3 were attached to the scapularneck. Two 50-mm cameras, positioned 60° apart, werecalibrated to record the 3-dimensional position of eachmarker. These data were recorded on computer foranalysis.

Sectioning Technique

The biceps anchor on the supraglenoid tubercle waslocated approximately 5-mm medial to the edge of theglenoid articular surface at the 12 o’clock position.Release of the biceps anchor was done sharply, takingcare not to violate the attachment of the superiorlabrum to the glenoid. The labrum was released bysharply elevating the labrum from the rim of theglenoid between the 10 and 2 o’clock positions whileavoiding extension into the biceps anchor. The anchorrelease was done in an outside-in fashion, whereas thelabrum was released in an inside-out fashion. Theorder of sectioning was randomized for each pair ofspecimens. After each stage of the sectioning proce-dure, the specimen was visually inspected and probedto assure that an appropriate release was performed.

Testing Protocol

Each specimen was mounted in a servo-controlledhydraulic material testing system (Bionix MTS, Min-neapolis, MN) before loading and aligned with theplane of the glenoid oriented perpendicular to thevertical actuator of the testing device. The rod secur-ing the biceps tendon was attached to the actuator inneutral rotation with the biceps tendon parallel to theactuator and perpendicular to the surface of the gle-noid.

Each specimen was tested in 3 phases. In eachphase, the tendon was loaded at 22 N/second to amaximum of 55 N, unloaded, and then allowed a2.5-second relaxation period before the loading cyclewas repeated. In the first phase, the intact specimenwas preconditioned with 25 loading cycles, followedby 10-second relaxation and then loaded with a singlecycle, during which displacement was recorded. PhaseII was initiated immediately thereafter with sectioningthe intact specimen at either the anchor or superiorlabrum origin of the LHBT. The sectioned specimenwas then loaded for a single cycle and data werecollected. The third phase immediately followed andinvolved sectioning the remaining portion of the ori-gin, in effect achieving a complete superior release.The specimen was then tested in the same manner.

The MTS and infrared photometric data were ana-lyzed by computer. Load versus displacement curveswere generated for all 3 runs for each specimen.Stiffness values were calculated as the rate of elonga-tion of the specimen during the 20 to 40 N loadingrange. This interval was selected because the loadingresponse was consistently linear in this range. Datawere normalized for pairs and sides with comparisons

379BIOMECHANICS OF THE BICEPS TENDON ORIGIN

made between intact, labral sectioned, and anchorsectioned specimens.

The increase in separation between a fixed point onthe glenoid and a point on the proximal portion of thebiceps tendon was recorded as displacement duringtesting and determined by the infrared photometricsystem. The displacement changes seen were evalu-ated under static and dynamic conditions. Static dis-placement was defined as the increased distance be-fore the test load was applied. This represented theincreased laxity or “slack” created by sectioning. Dy-namic displacement was defined as the maximumincreased distance measured at the 55-N load.

RESULTS

Stiffness

The average stiffness of the LHBT origin for the 14intact specimens was 103 N/mm (range, 61 to 311N/mm). Sectioning the anchor first resulted in a 52%reduction (P5 .003) in average stiffness and section-ing the labrum second resulted in a further decrease of20% (P 5 .04), resulting in a construct that was onaverage 72% less stiff than the intact specimen. How-ever, sectioning the labrum first caused only a 15%reduction (P5 .654) in average stiffness and section-ing the anchor second caused a further decrease of62% (P 5 .005), yielding a construct that was onaverage 77% less stiff than the intact preparation(Fig 1).

In specimens with both labrum and anchor originsreleased, the remaining attachment of the LHBT ori-gin to the glenoid was the anterior and posterior la-brum inferior to the 2 and 10 o’clock positions. Inthese completely released specimens, stiffness did notvary significantly with order of sectioning, measuring28% of the intact state when the anchor was sectionedfirst and 23% when the labrum was sectioned first.

Displacement

Maximal displacement at the 55-N load averaged0.99 mm for the 14 intact specimens. This dynamicdisplacement increased 101% with isolated sectioningof the biceps anchor (from 1.05 to 2.11 mm,P 5.004). Secondarily detaching the superior labrum ledto a further increase of 82% (from 2.11 to 3.83 mm,P 5 .05). The completely released specimen after thissectioning sequence had an average maximum dis-placement 365% that of intact preparations.

Isolated release of the superior labrum, however,resulted in only a 6% increase in average dynamic

displacement (from 0.93 to 0.99 mm,P 5 .604).Subsequent release of the biceps anchor resulted in a242% increased dynamic displacement (from 0.99 to3.39 mm,P 5 .002), yielding specimen with averagemaximum dynamic displacement 364% that of theintact preparations (Fig 2).

Static displacement did not change significantly,regardless of sectioning order, until both componentswere sectioned. Releasing either of the 2 origins re-sulted in less than 1 mm of increased average initialdisplacement, whereas releasing both resulted ingreater than 2 mm of increased average initial dis-placement.

DISCUSSION

Although relatively uncommon injuries, there havebeen numerous reports describing superior labral pa-thology and its treatment. Snyder et al.2 coined theterm SLAP for lesions of the superior labrum thatextended anteriorly and posteriorly. They developed awidely used classification system that consists of fourtypes: type I, isolated fraying of the superior labrum;type II, detachment of the superior labrum; type III, abucket-handle tear of the superior labrum; and type

FIGURE 1. Stiffness of the LHBT origin. (A) Sectioning thesupraglenoid origin first, and then releasing the superior labrum.(B) Releasing the superior labrum first, and then sectioning thesupraglenoid tubercle origin.

380 J. H. HEALEY ET AL.

IV, a bucket-handle tear extending into the LHBT.Type I SLAP lesions are thought to be senescentchanges, whereas types II, III, and IV lesions areconsidered pathologic or significant.6 The incidence ofthese significant SLAP lesions found at shoulder ar-throscopy has been reported to range from 5% to13%.6-8 The type II lesion, as initially described, in-cluded superior labral fraying, stripping of the supe-rior labrum, and incompetence of the biceps anchorwith arching of the biceps/labral complex away fromthe glenoid.

However, there is considerable variation in labralanatomy, and separating normal variation from truepathology can be difficult. Cooper et al.9 describedvariable sized sublabral synovial recesses, variationsin anterior labral attachment to the superior or middleglenohumeral ligament, and sublabral communica-tions with the subscapularis recess. Williams et al.10

described a benign variation of anterior labrum andmiddle glenohumeral ligament anatomy called the Bu-ford complex. This consisted of a detached anterosu-perior labrum that was continuous with a thick, cord-like middle glenohumeral ligament. Detrisac andJohnson11 found variations of the superior glenoid

labrum morphology, including a meniscoid variantwith a mobile and free inner edge.

There have been relatively few anatomic studiesspecific to the superior glenoid labrum and the LHBTorigin. In a cadaver study of 25 shoulders, Pal et al.12

reported that the primary origin of the biceps was thesupraglenoid tubercle in 25% of specimens and theposterosuperior labrum in 67%. More recently, Vangs-ness et al.13 studied 100 shoulders with histologicsectioning. They noted that in each specimen, 40% to60% of the LHBT fibers arose from the superiorglenoid labrum with the remainder from the supragle-noid tubercle.

There has been some research into the effect of thetype II SLAP lesion on shoulder biomechanics. Pag-nani et al.4 showed that a detachment of the superiorlabrum produced increased anterior and inferior gle-nohumeral translation and Rodosky et al.5 used adynamic model in an elaborate study demonstratingthat the LHBT/superior labrum complex contributedto anterior shoulder stability.

In the present study, a static shoulder model wasused to create a type II SLAP lesion by sequentialsectioning of the biceps anchor and stripping of thesuperior labrum. As mentioned above, Vangsness etal.13 have shown that the LHBT origin is anatomicallyequally shared by the labrum and the superior glenoidtubercle. However, this does not necessarily mean thatthe biomechanical properties of the 2 origin sites areequal.

The pathologic components of the type II SLAPlesion have been presented previously as involvingdetachment of the labrum and biceps anchor. Bey etal.14 recently presented a biomechanical cadavermodel for creating SLAP lesions. They applied tensileforce to the LHBT in intact cadaveric shoulders tostudy the mode of creation of a SLAP lesion. How-ever, they did not evaluate the biomechanics specificto each component of the origin of the LHBT.

Our findings reveal that the biceps anchor is theprimary restraint and that the superior labrum is asecondary restraint in regard to stiffness of the LHBTorigin and its displacement under significant load.However, average static displacement (i.e., with min-imal load) increased more than 1 mm only when bothcomponents of the origin were sectioned.

A possible limitation of this study is the arbitrarystatic orientation selected for testing. The model isartificial in that the humerus was disarticulated and,therefore, the LHBT was no longer in the intertuber-cular sulcus. Our primary interest was to evaluate thedisplacement of the biceps/labrum complex to tensile

FIGURE 2. Maximal displacement of the LHBT origin at a load of55 N. (A) Sectioning the supraglenoid origin first, and then releas-ing the superior labrum. (B) Releasing the superior labrum first,and then sectioning the supraglenoid tubercle origin.

381BIOMECHANICS OF THE BICEPS TENDON ORIGIN

forces. To meet this goal, we chose a model andorientation that could be uniformly reproduced in allspecimens and allowed access to the structures beingsectioned or evaluated. Normally, the orientation ofthe LHBT with respect to the glenoid varies withshoulder position. However, to our knowledge, nostudy has evaluated the orientation of the LHBT to theglenoid in various shoulder positions. The perpendic-ular orientation of the biceps to the glenoid in thismodel was used for reproducibility.

The magnitude of the force imparted on the LHBTin this study has been used previously.4 In a dynamicshoulder model, Rodosky et al.5 used LHBT forces ashigh as 160 N based on their estimates of maximalbiceps force. This higher force was derived from aformula that included measuring maximal cross-sec-tional area of the muscle and extrapolating electro-myographic data from pitchers reported by Gowan etal.3 We elected to use 55 N as the maximum force.Our interest was in displacement of the origin atclinically relevant loads that could potentially be ap-plied during arthroscopic assessment and not neces-sarily maximal predicted loads.

In our model, sequential release displacement val-ues and stiffness values show the biceps anchor as theprimary restraint and the superior labrum attachmentas the secondary restraint to tensile forces in theLHBT. We did not specifically study a true type IISLAP lesion, but a model we believe approximatesthis clinical entity. Based on our findings, it appearsthat both anchor and labrum detachments would benecessary to produce the clinical findings seen in atype II SLAP lesion

CONCLUSIONS

(1) The supraglenoid tubercle attachment (also re-ferred to as the biceps anchor) is the primary restraintto tensile forces at the origin of the LHBT. (2) Isolateddetachment of the superior labrum does not producesignificant changes in stiffness or displacement of theorigin of the LHBT. However, the superior labralattachment of the LHBT appears to act as a secondaryrestraint. (3) Detachment of both components is likelyif the superior labrum can be significantly displaced

with minimal force, as is typically seen in a type IISLAP lesion at arthroscopy.

Acknowledgment: The authors thank Jerry Alexander,Emir Kamaric, Vibor Paravic, and Mirena Paravic for theirvaluable contribution to this project.

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382 J. H. HEALEY ET AL.