stem damage during implantation of modular hip prostheses

564 THOUGHTS AND PROGRESS

Artif Organs, Vol. 30, No. 7, 2006

tricular function in endotoxin-induced shock in the pig. Inten-sive Care Med 1992;18:235–40.

9. Stein B, Pfenninger E, Grunert A, Schmitz JE, Hudde M.Influence of continuous haemofiltration on haemodynamicsand central blood volume in experimental endotoxic shock.Intensive Care Med 1990;16:494–9.

10. Grootendorst AF, van Bommel EF, van Leengoed LA, Nab-uurs M, Bouman CS, Groeneveld AB. High volume hemofil-tration improves hemodynamics and survival of pigs exposedto gut ischemia and reperfusion. Shock 1994;2:72–8.

11. Lee PA, Weger GW, Pryor RW, Matson JR. Effects of filterpore size on efficacy of continuous arteriovenous hemofiltra-tion therapy for Staphylococcus aureus-induced septicemia inimmature swine. Crit Care Med 1998;26:730–7.

12. Ratanarat R, Brendolan A, Piccinni P, et al. Pulse high-vol-ume haemofiltration for treatment of severe sepsis: effects onhemodynamics and survival. Crit Care 2005;9:R294–R302.

13. Bellomo R, Tipping P, Boyce N. Continuous veno-venoushemofiltration with dialysis removes cytokines from the circu-lation of septic patients. Crit Care Med 1993;21:522–6.

14. Sander A, Armbruster W, Sander B, Daul AE, Lange R,Peters J. Hemofiltration increases IL-6 clearance in early sys-temic inflammatory response syndrome but does not alter IL-6 and TNF alpha plasma concentrations. Intensive Care Med1997;23:878–84.

15. van Deuren M, van der Meer JW. Hemofiltration in septicpatients is not able to alter the plasma concentration of cyto-kines therapeutically. Intensive Care Med 2000;26:1176–8.

16. Morgera S, Slowinski T, Melzer C, et al. Renal replacementtherapy with high-cutoff hemofilters: impact of convection anddiffusion on cytokine clearances and protein status. Am JKidney Dis 2004;43:444–53.

Blackwell Publishing IncMalden, USAAORArtificial Organs0160-564X© 2006, Copyright the Authors; Journal compilation © 2006, International Center for Artificial Organs and Transplantation? 2006307554567Original ArticleTHOUGHTS AND PROGRESSTHOUGHTS

AND PROGRESS

Received November 2005; revised February 2006.Address correspondence and reprint requests to Dr. Luca Cris-

tofolini, Laboratorio di Tecnologia Medica, Istituti OrtopediciRizzoli, Via di Barbiano, 1/10, 40136 Bologna, Italy. E-mail:[email protected]

Stem Damage During Implantation of Modular Hip Prostheses

*‡Elena Varini, †‡Luca Cristofolini, ‡Marco Viceconti, and ‡Francesco Traina

*Dipartimento di Elettronica, Informatica e Sistemistica, University of Bologna; †Dipartimento di Ingegneria delle Costruzioni Meccaniche, Nucleari,

Aeronautiche e di Metallurgia, University of Bologna; ‡Laboratorio di Tecnologia Medica Istituti Ortopedici

Rizzoli, Bologna, Italy

Abstract: Modular hip prostheses are commonly usedtoday. Dedicated surgical auxiliary instrumentation isindispensable. A correct coupling between the prosthesisand the instrument is necessary to avoid damage. Basedon a specific commercial design (AnCAFit, WrightCremascoli Ortho, Toulon, France), the aim of this workwas to assess if stem holders of modular prostheses caninduce any stem damage, and if so, to identify the fea-tures that provide minimal damage. Two different stemholder connector designs were investigated. The surfaceof the stem cavity was inspected to identify the damagecaused by the use of the two connectors after a simulatedsurgical handling. Both connectors induced some dam-

age, with significantly more damage being caused by asupposedly improved design. Scars due to contact withthe stem holder were observed on the stem cavity sur-face. This may be sufficient to trigger increased frettingdamage during service life, causing the production offretting debris. Key Words: Total hip arthroplasty—Modular prostheses—Neck modularity—Morse taper—Surface damage—Surgical instrumentation.

The possibility of adapting the geometry of theprosthesis to the joint morphology of the patient pro-vides more flexibility during primary surgery andsimplified revision procedures (1,2). This is particu-larly useful when the hip anatomy is badly affectedas in developmental dislocation of the hip or in post-traumatic arthritis (3) and when a mini-approach isused (4). In recent years, modular stems are increas-ingly used in total hip replacement. This kind of pros-thesis is generally assembled by taper locks that showa low rate of disassembly and interface failure (2,5).For instance, the AnCAFit (Wright CremascoliOrtho, Toulon, France) modular hip prosthesis fea-tures a modular neck (that allows selection of differ-ent lengths and angles) that is press fitted in a cavityin the stem.

Several problems have been clearly identified withimplant modularity. Evidence of corrosion and fret-ting at the modular junction has been observed(1,6,7). Periprosthetic osteolysis is frequentlyobserved in association with wear and fretting debris(7). Nevertheless, many new designs have beenrecently introduced in the market. While their bio-mechanical performance has been thoroughly inves-tigated, much less attention has been paid to theproblems related to the implantation technique.

The tool kit used for the implantation of this kindof femoral components is made up of instruments(like stem holders) characterized by special connec-tors to be inserted in the modular cavities. The cou-pling geometry must be precise and the connectorhas to be rigid enough to withstand and transfer thehigh loads exerted during stem press fitting. A poorlydesigned connector or a badly handled instrumentcan damage the surface of the coupling (Fig. 1a), sothat it could compromise the precision coupling withthe neck, modifying the pressure distribution atthe coupling interface and damaging the finish androughness of the matching surfaces. The surface fin-ish is crucial in achieving optimal coupling of taperconnectors and minimizing fretting and surface dam-age (8). All these factors could promote the genera-tion of possible sites of fretting and associatedproduction of potentially harmful debris (5,8).

This work was aimed to assess if stem holders caninduce any stem damage, and if so to identify the

THOUGHTS AND PROGRESS 565


connector features that provide minimal damage tothe stem. It is suspected that when such stem holdersare handled during surgery, the loads applied manu-ally or when press fitting the stem (with hammerblows) could induce undesired contacts and damageto the stem.

MATERIALS AND METHODS

All the tests were conducted on five cementlessmodular neck stems (AnCAFit, stem sizes 11–16).Two different types of stem holder connectors(Wright Cremascoli Ortho) were tested to quantifythe amount of damage. Both are designed to beinserted into the stem housing for the modular neck,and both are characterized by the same rectangularsection with the larger flat sides linked by circularconnections. Connector A (Fig. 1b) has a coupling4.1° tapered end made of solid Ti6Al4V alloy thatperfectly fills the stem neck housing. It also featuresa screw made of AISI316L steel that fits the threadedhole placed in the central part at the bottom of thestem slot to fasten the connector to the prosthesis.Connector B (Fig. 1c) has a different coupling geom-etry characterized by a constant section, where onlythe distal part is stably in contact with the stem slot.Its structure is made of AISI316L steel. It has fourcylindrical inserts made of poly(tetrafluoroethylene)(PTFE) at the edges between the larger sides and thecircular sides of the section. The screw mechanism tosafely fasten the connector to the stem is the same asin connector A.

Inspection of surface damageThe analyses were performed on the whole surface

of the stem slot in order to explore the surface tex-

ture. A stereoscopic microscope (Nikon SMZ-2T,Tokyo, Japan) was used with a digital camera (NikonCoolpix 995) to record any scratches, incisions, andany other kind of damage. A piece of graph paperwas acquired in each image to provide for dimen-sional scaling. The magnification used (10–30×) forthe analysis was sufficient to detect damage with adimension at least equal to the spacing of the machin-ing ridges (about 100 µ).

Before each test and between each phase of thefollowing protocol, the stems were thoroughly ultra-sound clean washed:

1 A first surface inspection was performed on eachstem cavity in its original condition to record theinitial damage caused by manufacturing.

2 Then, a simulation of possible surgeon maneuversto completely press fit the stem into a femur wasperformed: the stem was first clamped in a vicewith rubber pads; connector A was inserted intothe stem cavity and properly fastened by the ded-icated screw. Ten axial hammer blows (hammerweight: 500 g) were applied. Additionally, cali-brated loads were applied by means of a handlelinked to the connector to reach 15–20 Nm oftorque (10 repeats). This type of load was agreedto by three experienced surgeons to simulate theworst possible clinical scenario. The torque wasapplied to simulate possible further applicationsfor intraoperative stability measurements (9).

3 The stem and the connector surfaces wereinspected with the same protocol.

4 After the microscopic inspection and documenta-tion of the damage caused by connector A, thesame stem was used for testing connector B, fol-

FIG. 1. Photographs of the stem cavity, which houses the mod-ular neck (a), of connector A (b) and connector B (c): the twoconnectors are similar, except that B does not feature the taperangle of A (the same of the stem cavity), and has four PTFEinserts at the corners.

a b

c

FIG. 2. Example of damage caused by the use of connector B:a couple of slight lines observed in some stems and placed atthe ends of the rectilinear sides (a) and the damage observedon connector B after the first mechanical simulation on a stem(b).

a b

566 THOUGHTS AND PROGRESS


lowing the same procedure. As the preliminarystudy indicated that surface damage caused byconnector A was minimal, the very same speci-mens were tested with both connectors, so as toallow paired comparisons using modularities thatwere machined with exactly the same tolerance.

5 Thus, the damage caused by the use of connectorB was quantified analyzing the added damage withrespect to the one observed after the use of con-nector A.

Effectiveness of the PTFE insertsOne more test was performed on connector B: the

metal part of this connector was supposed not tocome in contact with the stem while used by thesurgeon, because of the four PTFE inserts. It wassuspected that the inserts could get squeezed underload, allowing the sharp metallic connector edges totouch the stem slot surface. By measuring the stemconnector electrical conductivity, it was possible toassess if there was contact between the two metallicsurfaces. For this test, connector B was carefullyinserted into one stem cavity, and the connectorscrew was removed. A multimeter (Mod. 27; Fluke,Everett, WA, U.S.A.) was used to measure stem neckinsulation, while an axial force and a torque wereapplied manually.

RESULTS

Inspection of surface damageThe microscopic analyses revealed the presence of

damage caused by manufacturing. This is normally

small in dimension (single scratches, less than 1-mmlong), very localized, and with a random direction.The handling damage, on the contrary, was caused bythe compression of the machining ridges, mainlyalong the stem slot axis. After the use of connectorsA and B, respectively, the stems revealed additionaldamage if compared to the preliminary results. Thedamage induced by connector B was more extensive(more numerous, longer, and deeper scars), and dif-ferent in pattern if compared to the one caused byconnector A (Table 1).

Effectiveness of the PTFE insertsThe resistance measured by the multimeter when

the load was applied on connector B dropped tosmaller than 1 Ω (PTFE electrical insulation waslarger than 30 MΩ). This test showed that even witha small axial force or a torque applied, the metal partof connector B gets in direct contact with the stem.

DISCUSSION

Based on the results obtained, the followinggeneral conclusions can be drawn and should beconsidered when designing and handling modularprostheses:

1 Stem holders can produce surface damage inde-pendently of connector design. If these surgicaldevices are intended to be matched with the stemmodularity, they could produce a possibly danger-ous damage of the coupling surfaces.

2 The damage observed could compromise the cor-rect coupling between the two parts, leading to thegeneration of debris as produced by fretting (5). It

TABLE 1. Damage caused by the use of the two different connector geometries

A. Stem damageLocation: type Presence after connector A Presence after connector B

Central part of the anterior or posterior flat side:areas with contact damage (machining grooves dent)

Average per stem:0.8 counts


Between the circular and the rectilinear sides:slight line in axial direction, along half the cavity depth



Ends of the rectilinear sides:twin slight lines (Fig. 2a) in axial direction along the cavity depth. None


Central part of the anterior or posterior flat side:oblique scratch possibly induced by connector screwduring insertion

Average per stem:0.5 counts None

Between the circular and the rectilinear sides:PTFE residue trapped in the machining grooves. None In 2 stems

Bottom of the cavity:circular damage. None

Average per stem:4 counts

B. Connector damageLocation: type Connector A Connector B

On the rims along the PTFE inserts and the distal sharp corners:damage on the connector (Fig. 2b) None

Observed after the first test with the connector

THOUGHTS AND PROGRESS 567


is known, in fact, that the fretting process isstrongly influenced by the distribution of pressurebetween the surfaces at the fretting interface andalso by the surface roughness and finish (8).Changing of at least one of these factors couldpossibly have a strong effect in terms of frettingdamage (1,6,7,10).

While it is not possible to quantify the amount ofdebris that could be produced because of the dam-aged coupled surfaces, the outcome is potentiallyextremely detrimental. In fact, as a result, the metal-lic wear generated by fretting could lead to differentcomplications, which could compromise the long-term outcome of the operation (7).

CONCLUSIONS

Our experimental results suggest that much moreattention must be paid to the stem holders used withmodular prostheses: a robust design should be soughtso that unexpected loads will not result in undesir-able stem damage. Additionally, the surgeons shouldlimit, if possible, a direct hammering of the stemholder that could damage the matching surfaces ofmodular components and possibly lead to increasedfretting.

Acknowledgments: We would like to thank thefollowing: Wright Cremascoli Ortho for donating the

test material, Luigi Lena for the illustrations, andPaolo Erani and Gianluca Bersaglia for the technicaladvice.

REFERENCES

1. Bobyn JD, Tanzer M, Krygier JJ, Dujovne AR, Brooks CE.Concerns with modularity in total hip arthroplasty. ClinOrthop Relat Res 1994;298:27–36.

2. Cameron HU. Orthopaedic crossfire—stem modularity isunnecessary in revision total hip arthroplasty: in opposition.J Arthroplasty 2003;18(Suppl. 1):101–3.

3. Noble PC, Kamaric E, Sugano N, et al. Three-dimensionalshape of the dysplastic femur: implications for THR. ClinOrthop Relat Res 2003;417:27–40.

4. Kennon RE, Keggi JM, Wetmore RS, Zatorski LE, Huo MH,Keggi KJ. Total hip arthroplasty through a minimally invasiveanterior surgical approach. J Bone Joint Surg Am 2003;85-A(Suppl. 4):39–48.

5. Baleani M, Viceconti M, Walcholz K, Toni A. Metallic weardebris in dual modular hip arthroplasty. Chir Organi Mov1997;82:231–8.

6. Collier JP, Mayor MB, Jensen RE, et al. Mechanisms offailure of modular prostheses. Clin Orthop Relat Res 1992;285:129–39.

7. Cook SD, Manley MT, Kester MA, Dong NG. Torsional resis-tance and wear of a modular sleeve–stem hip system. ClinMater 1993;12:153–8.

8. Collins JA. Failure of Materials in Mechanical Design: Analy-sis, Prediction, Prevention. New York: Wiley-Interscience,1993.

9. Cristofolini L, Varini E, Pelgreffi I, Cappello A., Toni A.Device to measure intra-operatively the primary stability ofcementless hip stems. Med Eng Phys 2006;28:475–82.

10. Urban RM, Tomlinson MJ, Hall DJ, Jacobs JJ. Accumulationin liver and spleen of metal particles generated at nonbearingsurfaces in hip arthroplasty. J Arthroplasty 2004;19(Suppl.3):94–101.

stem damage during implantation of modular hip prostheses

Documents