Fracture of posterior-stablized tibial insert in a Genesis knee prosthesis

Tibial Post Refracture in Posterior-Stabilized Total Knee Arthroplasty Following Isolated Tibial Insert Exchange

Background

We report 4 cases with a tibial post refracture following isolated tibial insert exchange in posterior-stabilized total knee arthroplasty at a single center.

Methods

In our institution, 27 cases (26 patients) underwent reoperation due to tibial post fracture in posterior-stabilized total knee arthroplasty between July 2008 and November 2020. Of these 27 cases, 4 (4 patients) tibial post refractures occurred at a mean follow-up period of 9.1 years.

Results

All 4 cases of tibial post refracture occurred in a group of 21 cases with isolated tibial insert exchange. There was no tibial post refracture in patients with a tibial revision surgery. The incidence of this complication following isolated tibial insert exchange was 19.0%. The mean elapsed time from tibial insert exchange to the diagnosis of a post refracture was 2.5 years (range: 1.1-6.6 years).

Conclusions

Isolated exchange of a tibial insert with excessive thickness as a treatment for tibial post fracture is associated with a high probability of tibial post refracture. The treatment method should be selected after a thorough analysis according to the cause of fracture.

Level of evidence

IV.

High rate of tibial post-fracture in highly cross-linked polyethylene compared to conventional polyethylene in posterior-stabilized total knee arthroplasty at a minimum 12-year follow-up

PURPOSE

The purpose of this study was to assess tibial post-fracture between highly cross-linked polyethylene (HXPE) and conventional polyethylene inserts in consecutive posterior-stabilized (PS) total knee arthroplasty (TKA) over a minimum 12-year follow-up period.

METHODS

Between January 2007 and June 2008, a consecutive series of 2446 primary TKAs was performed in 1478 patients at a single institution. The final cohort was classified into two groups (1559 in the HXPE group and 887 in the conventional group) based on the insert material used during the procedure.

RESULTS

The mean follow-up period was 13.1 years in the HXPE group and 13.4 years in the conventional group. All 16 cases of tibial post-fracture occurred in the HXPE group. This complication rate was 1.03%. The mean elapsed time from primary TKA to the diagnosis of post-fracture was 3.9 years (range 0.5-10.0 years). Ten cases required replacement with a thicker polyethylene insert, and six cases underwent tibial revision surgery. There were no cases of tibial post-fracture in the conventional group.

CONCLUSION

Tibial post-fracture is not a rare complication in PS TKA with HXPE. Therefore, the possibility of a tibial post-fracture should be considered if newly developed pain, acute subluxation, effusion, patellar clunking, or instability occurs in patients treated with PS Scorpio knee system TKA using an HXPE insert.

LEVEL OF EVIDENCE

IV.

Traumatic fracture of the polyethylene tibial post and cone in a posterior-stabilized total knee arthroplasty: A case report

INTRODUCTION AND IMPORTANCE

Very few reports addressed polyethylene tibial post and cone fractures requiring revision surgery. Up to our knowledge, this particular pattern of wear, involving 2 fracture areas simultaneously, has not been previously described.

CASE PRESENTATION

A 70 year-old female developed a fracture involving both the tibial insert post and cone after a minor trauma after undergoing posterior-stabilized total knee replacement 4 years prior.

CLINICAL DISCUSSION

This unique pattern of wear may occur even after minor trauma.

CONCLUSION

We recommend having a low threshold to investigate minor trauma in this patient population, even years after undergoing primary knee arthroplasty.

Nontraumatic fracture of the femoral condylar prosthesis in a total knee arthroplasty leading to mechanical failure

This paper reports a case of fatigue fracture of the femoral component in a cruciate-retaining cemented total knee arthroplasty (TKA). A 64-year-old man had undergone a primary TKA for osteoarthritis 10 years previously at another institution using the PFC-Sigma prosthesis. The patient recovered fully and was back to his regular activities. He presented with a history of sudden onset pain and locking of the left knee since the preceding three months. There was no history of trauma, and the patient was mobilizing with difficulty using crutches. Radiographs revealed fracture of the posterior condyle of the femoral prosthesis. Revision surgery was performed as an elective procedure revealing the broken prosthesis. The TC3RP-PFC revision prosthesis was used with a medial parapatellar approach. The patient recovered fully without any squeal. Mechanical failure of the knee arthroplasty prosthesis is rare, and nontraumatic fracture of the femoral metallic component has not been reported before.

Bilateral condyle fracture of tibial insert in mobile bearing total knee arthroplasty

We report a case of polyethylene insert breakage in a 45-year-old man after 3.5 years of cruciate retaining type mobile bearing total knee arthroplasty (TKA). Interstingly, both condyles of the polyethylene insert have fractured. The visual assessment done by stereoscopic microscope in the investigation report suggested that the fracture propagation was a result of cyclic loading and that the fracture was from the articular surface as a result of tibio-femoral and anteroposterior shear loading. The initial flexion-extension gap mismatch and/or specific Asian habits like kneeling or deep knee bending could have been the possible factors for over-stress for the insert causing this complication. After replacement of the broken insert and modification for daily activity preventing deep knee flexion, the patient obtained complete relief of previous symptoms. It remains unclear whether insert breakage was secondary to polyethylene insufficient design or to the polyethylene material fracture propagation.

Fractography and oxidative analysis of gamma inert sterilized posterior-stabilized tibial insert post fractures: report of two cases

BACKGROUND

Highly crosslinked ultra-high molecular weight polyethylene (UHMWPE) has shown success in reducing wear in hip arthroplasty but there remains skepticism about its use in Total Knee Replacement (TKR) inserts that are known to experience fatigue loading and higher local cyclic contact stresses.

METHODS

Two Legacy Posterior-Stabilized (LPS) Zimmer NexGen tibial implants sterilized by gamma irradiation in an inert environment with posts that fractured in vivo were analyzed. Failure mechanisms were determined using optical and scanning electron microscopy along with oxidative analysis via Fourier Transform Infra-Red (FTIR) spectroscopy.

RESULTS

Micrographs of one retrieval revealed fatigue crack initiation on opposite sides of the post and quasi-brittle micromechanisms of crack propagation. FTIR of this retrieval revealed no oxidation. The fracture surface image of the second retrieval indicated a brittle fracture process and FTIR revealed oxidation in the explant.

CONCLUSIONS

These two cases suggest that crosslinking of UHMWPE as a manufacturing process or sterilization method in conjunction with designs that incorporate high stress concentrations, such as the tibial post, may reduce material strength. Moreover, free radicals generated from ionizing radiation can render the polymer susceptible to oxidative embrittlement.

CLINICAL RELEVANCE

Our findings suggest that tibial post fractures may be the results of in vivo oxidation and low level crosslinking. These and previous reports of fractured crosslinked UHMWPE devices implores caution when used with high stress concentrations, particularly when considering the potential for in vivo oxidation in TKR.

How to treat a tibial post fracture in total knee arthroplasty? A systematic review

BACKGROUND

Posterior-stabilized TKAs, which use a polyethylene tibial post to articulate against a metal femoral cam, are used regularly. Reported complications are related to the patellofemoral articulation or the tibial post-cam mechanism. Fracture of the tibial post is an uncommon but disabling complication after posterior-stabilized TKA that requires operative treatment.

QUESTIONS/PURPOSES

The literature was reviewed to determine the frequency of tibial post fracture and address three questions: (1) Is there a specific prosthetic design or patient demographics in knees with a fracture of the tibial post? (2) What are the common presenting complaints and methods of diagnosis? (3) What methods of treatment have been used?

METHODS

A PubMed search of English language articles from February 1982 to April 2010 was performed and 20 articles, all Level IV studies, were identified.

RESULTS

One specific design of posterior-stabilized tibial post with a central screw hole had a 12.4% incidence of fracture. Tibial post fracture has been reported with other designs, but with an incidence of 1% or less. The most common presenting symptoms include effusion, instability, or patella clunk syndrome. The most common method of diagnosis was clinical examination followed by arthroscopic examination. Treatment with revision to a new tibial polyethylene liner generally has been successful at short-term followup.

CONCLUSIONS

Tibial post fracture is a relatively uncommon complication after posterior-stabilized TKA that usually is treated successfully with liner exchange. The low quality of available literature makes it difficult to recommend a specific treatment protocol.

Highly crosslinked polyethylene in posterior-stabilized total knee arthroplasty: in vitro performance evaluation of wear, delamination, and tibial post durability

Recent advances in total knee arthroplasty (TKA) include the development of highly crosslinked polyethylene (HXPE). To assess the suitability of using HXPE in posterior-stabilized TKA, knee simulator wear testing and a novel tibial post durability test were performed on a modern posterior-stabilized implant design with both conventional polyethylene (CPE) and HXPE materials. The laboratory testing reproduced clinically relevant wear and tibial post damage mechanisms. For the designs tested, wear volume was reduced by 67% to 75% for aged HXPE compared with aged CPE. Components of HXPE also demonstrated superior tibial post durability compared with the CPE design, despite the use of unaged material to represent best-case CPE tibial post strength. With appropriate design considerations, HXPE can be successfully incorporated into a posterior-stabilized TKA.

Cruciate-retaining TKA using a third-generation system with a four-pegged tibial component: a minimum 10-year followup note

BACKGROUND

A third-generation TKA system was designed to address problems encountered with earlier designs including a high rate of patellofemoral complications. At a minimum of 5 years, we previously reported survivorship of 98.7% using revision for any reason as the endpoint for a cohort that includes the patients described in this report. That cohort was unique in that a tibial component that uses four short pegs for fixation was used in a subset of patients undergoing cruciate-retaining TKA and the tibial and femoral components were precoated with polymethylmethacrylate.

QUESTIONS/PURPOSES

We now report the survival rate, postoperative function, complications, radiographic loosening, and osteolysis at a minimum of 10 years postoperatively.

PATIENTS AND METHODS

We retrospectively reviewed 161 patients who underwent 179 TKAs. The patella was resurfaced in all cases and all components were cemented. Patients were evaluated clinically and radiographically for evidence of component loosening and osteolysis. Forty of the 161 patients (with 44 TKAs) had died and eight patients (nine knees) were lost to followup. For the remaining 113 patients (126 knees), the minimum followup was 10 years (mean, 11.7 years; range, 10.0-13.9 years).

RESULTS

Survivorship at a minimum of 10 years postoperatively using revision for any reason and revision for aseptic loosening were 97.7% and 100%, respectively. Three knees were revised: one for infection, one for periprosthetic fracture, and one for postoperative arthrofibrosis. There were no reoperations for patellar component maltracking, wear or loosening, tibiofemoral polyethylene wear, osteolysis, or aseptic loosening. Hospital for Special Surgery scores improved from a mean of 47.7 (range, 23-78) preoperatively to 85.4 (range, 33-100).

CONCLUSIONS

This cruciate-retaining design had a low risk of implant failure or revision and the design changes eliminated the patellar failures seen with earlier iterations at up to 10 years.

LEVEL OF EVIDENCE

Level IV, therapeutic study. See the Guidelines for Authors for a complete description of levels of evidence.

Contact stress at the anterior aspect of the tibial post in posterior-stabilized total knee replacement

BACKGROUND

Retrieval studies have demonstrated polyethylene wear and deformation of the anterior aspect of the tibial post of posterior-stabilized total knee replacements. We are not aware of any study that has analyzed the effects of the design of the femoral notch and the anterior aspect of the tibial post of posterior-stabilized total knee replacements on contact area, stress, and location. The purpose of the present study was to determine the levels of contact stress generated in three posterior-stabilized total knee replacement designs when the femoral intercondylar notch impinges on the anterior aspect of the tibial post.

METHODS

The contact area, mean and peak contact stresses, and contact location at the anterior aspect of the tibial post were determined when a posterior force of 100 N was applied to a NexGen LPS Flex, Genesis II, and Scorpio NRG total knee replacement. Measurements were performed at -15 degrees, -10 degrees, -5 degrees, 0 degrees, and 5 degrees of flexion of the femoral component both in neutral and with 5 degrees of rotation of the tibial component. Each measurement was sequentially performed five times, and the data were compared within and between the designs with use of analysis of variance and a post-hoc Scheffé F test.

RESULTS

The NexGen LPS Flex exhibited two contact areas on the medial and lateral corners of the anterior aspect of the post, whereas both the Genesis II and Scorpio NRG exhibited a single horizontal band. The NexGen LPS Flex showed the largest total contact area of the three designs at -15 degrees, -10 degrees, and 5 degrees of flexion. The NexGen LPS showed the lowest mean contact stress at -15 degrees and 5 degrees but showed the highest peak contact stress at -5 degrees and 0 degrees. The Scorpio NRG showed the highest mean contact stress under all conditions and showed the highest peak contact stress at -15 degrees, -10 degrees, and 5 degrees. With 5 degrees of rotation of the tibial component, peak contact stress increased, relative to neutral, 1.2 to twofold (depending on the flexion angle) in the Genesis II design.

CONCLUSIONS

The mean and peak contact stresses were variable for all three designs and the test conditions, approaching and in some cases exceeding the compressive yield stress for polyethylene. The geometry of the metal femoral notch and polyethylene tibial post in the axial and sagittal planes significantly affected contact area, mean and peak stresses, and contact location at the anterior aspect of the tibial post.

Two stage fracture of a polyethylene post in a 9-year-old posterior-stabilized knee prosthesis: a case report

Introduction

Several cases of tibial post breakage are reported in the literature. To the best of our knowledge, only three cases of NexGen knee prosthesis (Zimmer, Warsaw, Indiana, USA) tibial post failure have been reported.

Case presentation

In November 1999, a 63-year-old Caucasian woman from Italy with a history of symptomatic left knee osteoarthritis underwent a total knee arthroplasty. In March 2008, while rising from a chair, she felt a sudden pain and instability in her left knee. She reported a fracture of the polyethylene post of the tibial insert. No malposition or malalignment of either the femoral or tibial components were identified. The polyethylene tibial insert was studied under light microscopy and scanning electron microscopy. The fracture was also noted to have occurred without any notable polyethylene wear.

Conclusion

Scanning electron microscopy revealed two different damage patterns that could be explained with a two-stage rupture of our patient's polyethylene post. This could have been caused by a non-optimal ligamentous balancing during first implant surgery. Her knee probably developed a varus instability that weakened the post, and then a posterior anterior stress finally broke the polyethylene.

Early nontraumatic fracture of the polyethylene tibial post in a NexGen LPS-Flex posterior stabilized knee prosthesis

Fracture of a polyethylene tibial post in a knee prosthesis after a total knee arthroplasty (TKA) is an uncommon but severe complication of polyethylene tibial post wear. A fractured tibial post results in an unstable and malfunctioning knee. Revision knee surgery is necessary to restore function of the affected knee. We report a case of a nontraumatic fracture of a tibial polyethylene post that occurred in a 63-year-old man with left-sided hemiparesis 3 years after he had undergone left TKA with a NexGen LPS-Flex total knee prosthesis. To our knowledge, this is the first report of a nontraumatic fracture to the tibial post with anterolateral to posterior breakage after a NexGen LPS-Flex TKA.

Ultra high molecular weight polyethylene: mechanics, morphology, and clinical behavior

Ultra high molecular weight polyethylene (UHMWPE) is a semicrystalline polymer that has been used for over four decades as a bearing surface in total joint replacements. The mechanical properties and wear properties of UHMWPE are of interest with respect to the in vivo performance of UHMWPE joint replacement components. The mechanical properties of the polymer are dependent on both its crystalline and amorphous phases. Altering either phase (i.e., changing overall crystallinity, crystalline morphology, or crosslinking the amorphous phase) can affect the mechanical behavior of the material. There is also evidence that the morphology of UHMWPE, and, hence, its mechanical properties evolve with loading. UHMWPE has also been shown to be susceptible to oxidative degradation following gamma radiation sterilization with subsequent loss of mechanical properties. Contemporary UHMWPE sterilization methods have been developed to reduce or eliminate oxidative degradation. Also, crosslinking of UHMWPE has been pursued to improve the wear resistance of UHMWPE joint components. The 1st generation of highly crosslinked UHMWPEs have resulted in clinically reduced wear; however, the mechanical properties of these materials, such as ductility and fracture toughness, are reduced when compared with the virgin material. Therefore, a 2nd generation of highly crosslinked UHMWPEs are being introduced to preserve the wear resistance of the 1st generation while also seeking to provide oxidative stability and improved mechanical properties.

Tibial post failures in a condylar posterior cruciate substituting total knee arthroplasty

In posterior-stabilized total knee arthroplasties, a femoral cam and polyethylene tibial post are commonly used to restore posterior stability after sacrifice of the posterior cruciate ligament. This article reports a high incidence of early tibial post failures in one design of prosthesis and examines the variables that may have contributed to such. Five hundred sixty-four consecutive posterior-stabilized total knees were implanted in 512 patients, using a total knee prosthesis with a polyethylene tibial post and femoral cam. Clinical and radiographic outcomes were measured at a mean follow-up of 40 months after surgery (range, 24-83 months). At follow-up, 70 knees in 62 patients (12%) had undergone revision surgery because of symptoms related to catastrophic failure of the tibial post.

Damages of the tibial post in constrained total knee prostheses in the early postoperative course - a scanning electron microscopic study of polyethylene inlays

BACKGROUND

Investigation of the risk of fracture of the polyethylene (PE) inlay in constrained total knee prostheses.

METHODS

Three unused and seven polyethylene inlays that had been implanted in a patient's knee for an average of 25.4 months (min 1.1 months, max 50.2 months) were investigated using scanning electron microscopy (SEM). All inlays were of the same type and size (Genesis II constrained, Smith & Nephew). The PE surface at the transition from the plateau to the post was analyzed.

RESULTS

The unused inlays had fissure-free surfaces. All inlays that had been implanted in a patient's knee already had distinct fissures at the front and backside of the post.

CONCLUSION

The fissures of the transition from the plateau to the post indicated a loading-induced irreversible mechanical deformation and possibly cause the fracture of the inlay.

Failure of a metal-reinforced tibial post in total knee arthroplasty

Failures of the polyethylene tibial post in posterior stabilized total knee arthroplasty are usually associated with pain and knee instability. We report an unusual presentation of a tibial post failure that occurred after the polyethylene insert was revised in a posterior-stabilized total knee. The tibial post on the revised insert broke off, exposing a metal reinforcing pin. During the resulting posterior subluxations of the tibia, the metal pin articulated against the cam on the femoral component, resulting in the generation of metallic debris in the knee joint.

A study of effectiveness of knee arthroscopy after knee arthroplasty

PURPOSE

The purpose of this study was to investigate the outcome of arthroscopy in painful knee arthroplasty without evidence of infection, fracture, wear, and component loosening or malposition that had been refractory to conservative treatment. In addition, a literature review of 498 cases (MEDLINE 1966 to 2002) was performed.

TYPE OF STUDY

Case series.

METHODS

From 1997 to 2000, 27 patients (20 women, 7 men) had undergone arthroscopies because of poor results following total knee arthroplasty. Before the operation, the patients had suffered symptoms for an average of 11 months (range, 3 to 41 months). The average onset of symptoms after knee arthroplasty was 26 months (range, 3 to 59 months). The average patient age was 70 years (range, 42 to 81 years) and the average follow-up was 34 months (range, 24 to 52 months). At the initial operation, 19 patients had received total condylar surface replacement and 8 had received hemireplacement. Patients were evaluated using the Knee Society rating system. A review of the literature was performed by initial identification of the articles from a MEDLINE database followed by the use of cross references.

RESULTS

All of the patients were available for follow-up. Eighteen of the 27 procedures resulted in an improvement in the patient's knee score. The average Knee Society ratings increased from 71 points before arthroscopy to 85 at follow-up for the knee score. The average functional scores were 69 and 83 points, respectively. The Knee Society pain score improved from 32 to 41 points. Nine patients underwent a subsequent open revision after arthroscopic diagnosis or treatment. Operative diagnoses included arthrofibrosis, impinging hypertrophic synovitis, impinging posterior cruciate ligament stump, prosthesis loosening or wear, symptomatic pseudomeniscus, an infrapatellar spur, and meniscal rupture. There was 1 infection as a complication associated with the arthroscopic procedure.

CONCLUSIONS

Arthroscopic treatment of painful knee arthroplasty provides reliable expectations for improvement in function, decrease in pain, and improvement in knee scores for most patients.

LEVEL OF EVIDENCE

Level IV, Case Series.

Sagittal laxity in vivo after total knee arthroplasty

INTRODUCTION

A stress arthrometry study of 77 knees undergoing total knee arthroplasty was performed to determine the difference in anteroposterior (AP) laxity between posterior cruciate ligament (PCL)-retaining (PCLR) and PCL-substituting (PCLS) prostheses using the Genesis I TKA.

MATERIALS AND METHODS

Fifty-three knees had PCLR and 24 had PCLS prostheses. The selected patients had successful arthroplasties after a minimum follow-up of 5 years. AP laxity was measured with a KT-2000 arthrometer (Medmetric, San Diego, CA, USA) using standard protocols.

RESULTS

At 30 degrees of flexion, there was no statistical difference in anterior (PCLR: 4.7 mm, PCLS: 4.5 mm), posterior (PCLR: 1.1 mm, PCLS: 0.7 mm), or total (PCLR: 5.8 mm, PCLS: 5.3 mm) displacement. At 75 degrees , significant differences were seen in both anterior (PCLR: 3.3 mm, PCLS: 2.3 mm) and total (PCLR: 4.8 mm, PCLS: 3.4 mm) displacement (p=0.001 and p=0.009, respectively), although there was no statistical difference in posterior displacement (PCLR: 1.5 mm, PCLS: 1.1 mm).

CONCLUSION

The above values are considered the suitable degree of AP laxity in total knee arthroplasty for a satisfactory clinical outcome 5-9 years after surgery. The PCL in a PCLR prosthesis and the central tibial spine and femoral cam in a PCLS prosthesis might play comparable roles in determining the laxity in the posterior direction in these prostheses.

Fracture of the polyethylene tibial post in a NexGen posterior-stabilized knee prosthesis

We reported a case of fracture of a polyethylene tibial post in a 44-year-old woman after 3 years of NexGen posterior-stabilized total knee arthroplasty (Zimmer, Warsaw, IN). Burnishing and delamination of the polyethylene was found around the breakage site of the post, especially over the anterior aspect of the post base. It indicated that the possible failure mechanism was the repeated anterior impingement between the metal femoral cam and polyethylene tibial post. After replacement of the broken insert, the patient obtained complete relief of previous symptoms. To our knowledge, this is the first report of post breakage of a NexGen posterior-stabilized knee prosthesis.

Secondary knee instability caused by fracture of the stabilizing insert in a dual-articular total knee

A case of a fractured polyethylene stabilizing insert causing secondary knee instability in a Dual-articular total knee arthroplasty (TKA) is presented. A 65-year-old woman who underwent surgery with a Dual-articular TKA 4 years earlier had a well-functioning prosthesis until a fall, after which she began complaining of an intermittent, audible clicking; pain; and instability. At surgery, a fractured polyethylene peg was discovered, and the tibial insert was replaced. At follow-up, she remains asymptomatic. This is the first description of a fractured stabilizing insert in a Dual-articular knee.

Coronal laxity in extension in vivo after total knee arthroplasty

We performed stress arthrometric studies on 77 knees (71 patients) with total knee arthroplasty to determine the presence and magnitude of femoral abduction and adduction in knee extension. A total of 53 knees (49 patients) had posterior cruciate ligament-retaining (PCLR) prostheses, and 24 (22 patients) had PCL-substituting (PCLS) prostheses. The selected patients had successful arthroplasties with no clinical complications a minimum of 5 years after primary surgery. Each patient was subjected to a successive abduction and adduction stress test at 0 degrees -20 degrees of flexion using a Telos arthrometer. The mean values for abduction and adduction were 4.8 degrees and 4.5 degrees with a PCLR prosthesis, respectively, and 4.6 degrees and 4.0 degrees with a PCLS prosthesis. There were no statistical differences between PCLR and PCLS knees. The results suggest that approximately 4 degrees of laxity in these directions is suitable in total knee arthroplasty for a satisfactory clinical outcome 5-9 years after surgery.

Recurrent dislocation of total knee arthroplasty: an unusual cause

We report an unusual case of recurrent dislocation of posterior stabilized total knee arthroplasty (TKA) secondary to fracture of the polyethylene insert. Recurrent dislocation occurred 21 months after primary TKA. Fractured polyethylene insert was noted as an abnormal radiolucent shadow on postoperative radiographs and was confirmed by an arthrogram. At revision surgery, disintegration of the insert, including the tibial post, was found. We recommend careful scrutiny of the integrity of the polyethylene insert at the time of follow-up evaluation in cases of TKA dislocation.

The causes of insert backside wear in total knee arthroplasty

Wear of the insert backside occurs ostensibly because of micromotion at the undersurface articulation that occurs with loading. When a cyclic axial load was applied to contemporary knee implants, all inserts tested moved 2 to 25 microm in the shear plane relative to the metal backing suggesting that undersurface motion may be inevitable. Variables that increase the forces between the insert and metal backing can worsen relative micromotion and backside wear. Forces at the undersurface articulation, created during physiologic loading, are influenced by insert type, articular design, and surgical technique. Increasing articular insert constraint can cause forces at the main articulation to be resisted and transferred to this and the other interfaces. Designs with a cam post mechanism that force rollback at a certain flexion angle create a significant force in this shear plane. Inserts with highly conforming articular geometries can have a similar affect if used to inhibit anteroposterior or mediolateral motion of the femur on the tibial insert. Component alignment and position, and ligament balance also may influence backside wear as suggested by the great variability of wear patterns seen on like insert retrievals and by kinematic differences observed in fluoroscopic studies of the same implant design. Only by understanding these potential causes of backside motion and subsequent wear, can backside wear be mitigated.