A unique, laterally stabilized design concept (3D Knee-DJO Surgical, Inc) for total knee arthroplasty (TKA) without traditional post and cam construct was developed to allow surgeons to resurface the arthritic knee while choosing to maintain or sacrifice the posterior cruciate ligament (PCL). Reported complications with current ‘post and cam’ designs of PCL-substituting TKRs include higher polyethylene wear associated with cam-post impingement, increased bone interface shear stresses, and more distal femoral bone resection making revisions more complex and problematic. The effectiveness of this laterally stabilized TKA design has been extensively studied biomechanically using both in-vitro and in-vivo methods. It was hypothesized that for this total knee arthroplasty design; the mid-term clinical, radiographic and functional results would be the same for patients having two different surgical techniques in which the posterior cruciate ligament was either completely retained or completely resected. This study reports on eight year clinical results as well as in-vivo fluoroscopic results and retrieval data. Reported are 159 patients with 116 knees done by a surgeon who preserved the PCL with a bone block technique and 43 knees by a second surgeon who completely resected the PCL. Clinical results did not statistically differ between the two groups and found Knee Society Scores of 96 for Pain and 91 for Function. Average ROM was measured at 124 degrees. Comparative fluoroscopic imaging analysis of in-vivo dynamic flexion activities of thirty-three (20 PCL-preserved and 13 PCL resected) knees was performed demonstrating stable performance and only small (non-significant) mechanical differences. Analysis of two unrelated groups of tibial polyethylene inserts, the first retrieved from patients after 1–4 years in-vivo function (n = 14) and the second after in-vitro knee wear simulation (n = 4) showed low wear rates with no delamination. There was only one failure for mechanical loosening in the cruciate resected group and radiolucent lines of greater than 2 mm were only seen in 4% with none being progressive. Kaplan-Meier Survivorship, using mechanical loosening as the end point, was 99.2% at an average of 8.8 years. In summary, this laterally stabilized TKR design offers a very good alternative to standard ‘post and cam’ PCL sacrificing TKRs while still giving surgeons the ability to maintain the PCL if desired.

Outcomes following TKA often are good, but patients sometimes lack adequate range of motion and strength. Reasons for these deficits may include instability and the loss of cruciate ligament function. One approach to TKA design is to retain the PCL, and configure the TKA surfaces to approximate the function of the ACL. This can be accomplished by having a lateral surface that controls tibiofemoral motion near extension, but allows femoral rollback with flexion. We have been using such a fixed-bearing TKA design since 2001. The purpose of this study was to determine if an ‘ACL-substituting’ arthroplasty design provides clinical and functional results comparable to traditional PCL-retaining arthroplasty designs.

This series consists of 407 consecutive knees in 185 male and 222 female patients (73±9 years, 28±5 BMI) operated from November 2001 to August 2006. All patients underwent TKA by the same surgeon using PCL-retention and implantation of the same cemented ‘ACL-substituting’ TKA design. Clinical outcomes were evaluated using Knee Society Scores and radiographic review for the first 100 TKA with minimum 2 year follow-up. A subset of patients participated in IRB-approved protocols to quantitatively evaluate TKA motion and strength. Functional outcomes were assessed during gait, stair-climbing and curb step-over tasks for 10 unilateral TKA using a motion capture system, force platforms and inverse dynamics to measure the dynamic knee joint flexion moment. Kinematic outcomes were studied during kneeling for 20 TKA using fluoroscopy and shape matching techniques.

Knee Society Scores averaged 96+7 (pain) and 95+12 (function) at an average of 3.2+0.7 (range, 2 to 5) years follow-up. Passive flexion averaged 122°±10°, with 70% of the TKA achieving > 120° flexion. Radiolucent lines (2–4 mm wide) were observed in 7 TKA. Peak flexion moments (dynamic strength) for the TKA averaged 79%, 80% and 85% of the patients’ contralateral normal knees during the gait, stair-climbing and step-over tasks, respectively. In maximum kneeling, knees averaged 131°±13° flexion, 10° ±4° tibial rotation, and 2mm/10mm posterior position of the medial/lateral condyles.

This series’ early clinical follow-up was comparable to any well performing TKA. Knee flexion during passive examination and kneeling were comparable to the best reported results for PCL-retaining and PCL-substituting TKA. Peak knee flexion moments, a measure of functional strength, were comparable to the strongest knees reported in the literature. These early results suggest a fixed-bearing prosthesis with ‘ACL-substitution’ can provide patient performance comparable to the best performing designs.

Numerous fluoroscopic studies of total knee arthroplasty (TKA) kinematics have shown that many contemporary TKA designs exhibit abnormal tibiofemoral translations during activities like gait and stair climbing. One reason for these abnormal motions is the absence of the anterior cruciate ligament (ACL) in the vast majority of knees with TKA. The purpose of this study was to analyze knee kinematics during gait and stair activities in patients with a new design of TKA, incorporating a lateral compartment which is fully congruent in extension, but lax in flexion approximating the function of the anterior cruciate ligament. Our goal was to determine if such ACL-substitution results in more normal weight-bearing kinematics during gait and stair activities.

Thirteen ACL-substituting TKAs (AS knees) in 8 patients were observed using fluoroscopy during treadmill gait (1 m/s) and stair stepping. Model-image registration was used to determine the 3D knee kinematics. These kinematics were compared with those from 5 knees with posterior cruciate preserving TKA (PCL Group) and 7 knees with ACL-intact bi-unicondylar arthroplasties (bi-UNI Group). AS Group subjects were 12±6 months post-op. Control groups (PCL Group/bi-UNI Group) subjects were 72±6/15±6 months post-op.

During gait, the AS knees showed 1.6±0.4mm medial condyle posterior translation from heel strike to the middle of stance phase and 2.6±0.3mm posterior translation during swing phase. A similar pattern was observed in the bi-UNI knees. The lateral condyle translated posteriorly 2.1±0.2mm from heel strike to terminal stance phase, similar to the PCL knees and the bi-UNI knees. The center of rotation was predominantly lateral (19% lateral) from heel strike to mid-stance and then moved medially (16% medial) in swing phase. AS knees showed 3.4°±2.4°of internal tibial rotation from mid-stance to terminal stance, similar to the bi-UNI knees. During the stair activity, medial/lateral condylar AP translation in the AS Group was 1.6±0.1mm/2.0±0.3mm from extension to flexion, similar to the bi-UNI knees. The AS knees showed 5.9°±2.4° of internal tibial rotation from 20° to 80° during stair activity, similar to the bi-UNI knees.

Substitution of the ACL by a lateral compartment which is conforming in extension may provide more natural stability and function with knee arthroplasty. Medial condylar translations and axial rotations were similar to those observed in ACL-intact bi-unicondylar knees. Gait kinematics were similar to those reported for healthy natural knees [Koo S and Andriacchi TP, J Biomechancs, 2008]. The long-term success of TKA depends not only on kinematic factors, such as those reported here, but also on polyethylene wear and patellar complications. A longer-term clinical study will be required to determine if ACL-substituting TKA represents an overall functional and clinical improvement compared to more traditional designs.

There is interest to provide total knee arthroplasty (TKA) patients large ranges of functional knee flexion. Factors contributing to flexion include a posterior femoral position on the tibia, posterior condylar offset, and posterior tibial slope. These factors can be incorporated into implant designs and surgical techniques. It is useful to assess the robustness of the resulting design, that is, the consistency of kinematic or functional results when patient and surgical factors vary widely. This study evaluates in vivo flexion performance of a single implant design in patients whose posterior cruciate ligament (PCL) was either retained or sacrificed.

28 knees in 20 patients were imaged using fluoroscopy during maximum flexion kneeling and lunge activities. 20 knees (12 patients) received TKA with the PCL retained by a bone block (PCL+ group). Eight knees (7 patients) received TKA with complete PCL resection (PCL- group). All knees received a fixed-bearing TKA (3D Knee^™, Encore Medical, Austin, TX) with an asymmetric tibial bearing having a sagittally curved medial compartment and a lateral compartment fully congruous with the lateral condyle in extension (approximating anterior cruciate ligament substitution). Three-dimensional knee kinematics were determined using model-based shape registration techniques.

For the kneeling activity, mean implant flexion was 124°±11° for PCL+ knees and 121°±17° for PCL- knees (p> 0.05), mean tibial internal rotation was 10°±4° for PCL+ knees and 9°±3° for PCL- knees (p> 0.05) and tibial valgus was −1°±1° for PCL+ knees and 2°±4° for PCL- knees (p=0.003). Medial contact location averaged −2±4mm and for PCL+ knees and −1±2mm for PCL- knees (p> 0.05). Lateral contact location averaged −10±4mm for PCL+ knees and −7±1mm for PCL- knees (p> 0.05). For the lunge activity, mean implant flexion was 120°±11° for PCL+ knees and 121°±21° for PCL- knees (p> 0.05), mean tibial internal rotation was 11°±4° for PCL+ knees and 8°±3° for PCL- knees (p> 0.05) and tibial valgus was −1°±1° for PCL+ knees and 2°±2° for PCL- knees (p=0.0002). Medial contact location averaged 0±4mm for PCL+ knees and −4±3mm for PCL- knees (p=0.04). Lateral contact location averaged −8±4mm for PCL+ knees and −9±4mm for PCL- knees (p> 0.05).

There was no difference in implant flexion between PCL retaining and sacrificing TKA. Both groups had knees with more than 145° implant flexion (~155° skeletal flexion). There were no significant differences in tibial rotation or lateral condylar contact locations. There were differences in tibial valgus for both activities. PCL- knees exhibited a tendency for the medial compartment to ‘book open’ with flexion beyond 130°, consistent with loss of PCL function. Based on this small cohort comparison, it appears that robust flexion performance and knee kinematics can be obtained with a fixed-bearing TKA design.

Survivorship of unicondylar knee replacement (UKR) exceeds 85% at 10 years. During long term follow-up, progressive osteoarthritis (OA) and loosening are typical of UKR failure. The decision to revise UKR is complex as radiographic findings are not always consistent with clinical symptoms. This study of revised UKR compares intraoperative assessment of component fixation and progressive OA with prerevision radiographic evaluations.

Twenty-seven UKR were retrieved from 22 female and 5 male patients. Patient age and time in situ averaged 76 (68–87) years and 79 (25–156) months, respectively. At index arthroplasty, all knees received a fixed-bearing medial UKR with cement fixation. Prior to revision, radiolucent lines and component alignment were assessed on radiographs according to Knee Society guidelines. Suspected revision reasons based on clinical and radiographic evaluation included aseptic loosening (63%), progressive OA (22%), and wear (15%). During revision surgery, component fixation was manually assessed and graded as well-fixed or loose, and progressive OA was graded using Outerbridge classification. Intraoperative and radiographic assessments were completed independently.

Average Knee Society Scores declined > 30 points to 53+18 (pain) and 43+11 (function) before revision. During revision surgery, femoral and tibial component fixation were graded as loose in 19 (70%) and 9 (33%) knees, respectively. There was Grade III or IV progressive OA in the lateral or patellofemoral compartment of 15 (56%) and 16 (59%) knees, respectively. Radiolucent lines were evident in 8 of 19 loose femoral components and 5 of 9 loose tibial components. In contrast, 3 of 8 well-fixed femoral components and 6 of 18 well-fixed tibial components had radiolucent lines. There were 11 loose femoral components and 4 loose tibial components without radiolucent lines. Radiographic limb alignment averaged 3°+3° valgus immediately after index UKR. Change in limb alignment ranged from 0° to 17° at revision. Tibial or femoral component alignment changed 5° to 9° in 12 (44%) knees and > 10° in 5 (19%) knees. Eight of these 17 knees (47%) had malaligned components graded as loose.

The prevalence of progressive OA at revision UKR was more than double occurrence suspected from radiographs. Interpreting radiographic indications for loosening was difficult. Radiolucent lines predicted loosening in 46% (13/28) of the components graded as loose and falsely predicted loosening in 35% (9/26) of the components graded as well-fixed. Radiolucent lines were absent in 15/28 (54%) of the loose components and changes in component alignment > 5° were associated with component loosening in < 50% of the knees. Rigorous attention to clinical symptoms and careful interpretation of radiographic phenomena are needed to determine indications for revision in UKR patients.

Achieving normal strength after total knee arthroplasty (TKA) remains a major challenge, with recent reports suggesting strength following TKA averages 65% of normal. Gait lab studies have reported the greatest strength (80%–84% of normal) in knees with intrinsic stability, i.e. knees where the implant surfaces or retained ligaments provide definitive control of tibiofemoral motions such that dynamic muscle stabilization is not required. Superior results have been reported for bicruciate retaining arthroplasty, posterior-stabilized arthroplasty with early-engaging cams, and single radius highly congruent posterior-stabilized arthroplasty. The goal of this study was to determine if knees with an intrinsically stable posterior cruciate ligament (PCL) retaining implant design showed strength comparable to these other intrinsically stable types of arthroplasty.

Ten patients with unilateral intrinsically stable PCL-retaining knee arthroplasty were studied using full-body motion capture, force platforms and electro myography while they stepped onto and over a 20cm box. The implant design includes an asymmetric tibial bearing surface with a fully congruent lateral articulation (0°–70° flexion). Subjects were recruited on the basis of combined KSS scores greater than 180 one year after surgery. Peak knee flexion torques, normalized by body weight and height, are used as a measure of functional strength.

Knees with posterior cruciate retaining, intrinsically stable TKA exhibit functional strength comparable to other intrinsically stable TKA designs and superior to strength in less stable TKA designs. These knees show some reduction of knee flexor activation, indicating that antagonist coactivation is not required for joint stability. Stable joints permit more optimal muscle activation, making patients effectively stronger while reducing loads at the joint.

Contemporary knee implants use a variety of methods to control tibiofemoral motions. Posterior stabilized implants have a post and cam to force the femur posterior with flexion. Most posterior cruciate retaining designs rely solely on this ligament and symmetric tibial surfaces to control tibiofemoral translations. However, many studies have demonstrated poor control of tibiofemoral motion in PCL retaining knees. One strategy to augmenting PCL function is to provide a gait-congruent lateral articulation providing definitive stability in extension while allowing lateral condylar translation in deep flexion. It is unknown whether this design strategy, essentially substituting for the ACL, allows the PCL to function more normally.

Fifteen knees in ten patients with a fixed-bearing, PCL retaining, lateral pivot arthroplasty were observed during maximum flexion kneeling and lunging using fluoroscopy. The tibial insert provides a fully conforming lateral articulation from 0°–70° flexion, allowing lateral AP translation at greater flexion. Recruited on the basis of combined KSS scores > 180 points, patients averaged 72 years, 27.5 BMI, and 12 months post-op. Shape matching techniques were used to determine the 3D pose of the implant components.

Skeletal flexion during kneeling averaged 134° (117°–156°) with 11° tibial internal rotation. Medial condylar contact was 3mm posterior, and lateral contact was 11 mm posterior to the tibial AP midpoint. Skeletal flexion during lunging averaged 122° (106°–146°) with 11° tibialinternal rotation. Medial condylar contact was 1mm posterior, and lateral condylar contact was 9mm posterior to the tibial AP midpoint.

Knees with lateral pivot arthroplasty exhibited flexion comparable to the best reported results in North American patients. Tibial rotation was statistically greater than has been reported for symmetric posterior stabilized or PCL retaining implants for the same activities. Posterior translation of the condyles with flexion beyond the range of full articular congruity is consistent with relatively normal PCL function.

There is renewed interest in unicondylar knee replacements (UKR) to meet the increasing demand for less invasive surgical procedures for knee arthroplasty. UKR survivorship exceeds 85% at 10 years, with unconstrained (round-on-flat) designs showing significantly better survivorship than conforming designs. However, round-on-flat articulations have the potential for poor wear performance and more conforming, mobile-bearing UKR designs have been advocated. This study evaluates the wear performance of unconstrained UKR polyethylene bearings retrieved at revision knee arthroplasty.

Forty-two UKR with fixed polyethylene tibial bearings were retrieved. Patient age and time in-situ averaged 73 (45–89) years and 7 (1–19) years, respectively. All knees had intact cruciate ligaments at index surgery. Revision reasons included loosening (45%), progressive arthritis (17%), polyethylene wear (17%), instability (5%), and other (17%). Retrospective radiographic review of radiolucent lines and component alignment was completed using Knee Society guidelines. Polyethylene articular damage size (% of articular surface area), location and damage mode incidence were measured using microscopy and digital image analysis.

Damage area was centrally located and averaged 65%+22%. The largest damage areas consisted of abrasion (19%) and scratching (17%). Revision for loosening or wear was significantly correlated with greater damage area (Spearman Correlation, p=0.049). The incidence of scratching, pitting and abrasion each exceeded 70%, including 29 inserts with peripheral abrasive damage consistent with impingement between the polyethylene and extra-articular cement or bone. Anterior damage location and abrasion were significantly correlated with component position (p< 0.001). Concave surface deformation due to femoral component contact was externally rotated (24 inserts), consistent with tibial external rotation relative to the femoral component, neutrally aligned (11 inserts), internally rotated (4 inserts), and indeterminate (3 inserts).

Despite initial tibiofemoral incongruity and concerns of high contact stress, round-on-flat UKR offers a durable knee arthroplasty. The relatively unconstrained tibiofemoral articulations allowed freedom of placement on the resected bone surfaces and a range of tibio-femoral rotation during activity, as demonstrated by the rotated concave surface deformations. Such deformation may reduce polyethylene contact stresses by increasing the tibio-femoral contact area. However, similar to retrieved mobile bearing UKR which show a 63% incidence of impingement, abrasive damage on these fixed bearing UKR has consequences for polyethylene debris generation and the transmission of shear forces to the bone-implant interface. Rigorous attention to conventional and minimally invasive surgical technique, including cement fixation and component position, is needed to reduce the incidence of abrasive polyethylene damage.

Introduction: Full flexion is a critical performance requirement for patients with total knee replacement (TKR). Different design strategies, such as the post-and-cam, are used to achieve greater femoral rollback during knee flexion. However, substantial damage to the polyethylene tibial post on some posterior cruciate ligament substituting (PS) TKR designs has led to concerns that femoral camtibial post contact will lead to increased insert micromotion and backside wear in modular PS TKR designs. This study evaluated in vivo knee function and polyethylene wear in patients with posterior cruciate ligament retaining (CR) and PS tibial component designs with a full peripheral rim modular locking mechanism.

Methods: Motion Analysis: Thirty two knees with CR (9 knees) and PS (23 knees) tibial inserts participated in fluoroscopic motion analysis during activities of daily living, including stairrise/descent, treadmill gait and maximum kneeling flexion. The metal tibial components used the same full peripheral rim locking mechanism design with the different modular polyethylene articular surfaces. Tibial-femoral contact locations were determined throughout the full range of motion for all activities.

Retrieval Analysis: Polyethylene tibial inserts were retrieved during autopsy and revision surgery after 1 to 74 months. There were 37 CR inserts and 7 PS inserts of the same designs that were evaluated in the motion study. Backside damage on the inserts was assessed on all retrieved inserts using optical microscopy and the damage area and location was measured using digital image analysis.

Results: A relatively posterior position of the femoral component on the tibia was significantly correlated with greater maximum knee flexion. PS TKR had significantly more posterior femoral position and greater maximum flexion than CR TKR. The mean backside damage area was 38%+10% for PS inserts and 45%+15% for CR inserts. Backside surface damage was concentrated near the inserts’ peripheral rim and was dimpled in appearance, consistent with a cast impression of the textured metal baseplate. Scratches and burnishing was infrequently observed. Inserts with the greatest area of backside damage were in-situ for the longest time period.

Discussion: This fluoroscopy-based motion analysis study showed that knees with PS TKR achieve greater maximum flexion than knees with CR TKR. However, retrieved PS inserts did not have larger backside damage areas and the damage pattern location was consistent for both articular geometries. The observed damage morphology suggests that backside damage resulted from axial compression of the polyethylene insert against the textured baseplate rather than micromotion. Previous mechanical tests of this same modular tibial component design have shown that motion between the polyethylene insert and metal baseplate does not increase even after more than six years of in-vivo function.

Introduction: Patellar complications after total knee arthroplasty (TKA) remain a common reason for failure. Fully congruent patellar components, with larger contact areas and a polyethylene articular surface that is free to rotate in the frontal plane (LCS design), were designed to accommodate patellar mechanics and decrease wear. However, it remains to be determined whether the perceived advantages of a mobile-bearing, fully congruent patella design are realized in-vivo. The purpose of this study is correlate wear patterns on congruent mobile-bearing patellar components with patellar mechanics that existed after TKA.

Methods: Uncemented metal-backed patellar components were retrieved at revision surgery from 26 knees with meniscal bearing (23 knees) and rotating platform (3 knees) LCS mobile bearing prostheses (DePuy Orthopaedics). Mean patient age and time in-situ was 75+11 years and 11+4 years, respectively. Revision reasons included bearing wear (11), patella wear (7), instability (2), pain (3), loosening (1), osteolysis (1), and unknown (1). Polyethylene damage was assessed using optical microscopy. Articular wear area was measured using digital image analysis and the % area, location and incidence of each damage mode was calculated.

Results: The average damage area on the retrieved patella occupied 69%+15% of the surface. Burnishing, delamination and scratching modes occupied the largest areas. Delamination was noted on 58% of the retrieved patellae, predominantly located in the superior-medial quadrant. Nine (35%) patellae were fractured, with the fracture plane typically oriented in the medial-lateral direction or along the lateral edge. Twently one (81%) patellae had subsurface cracks oriented along the superior-inferior axis on the extreme lateral edge and along the medial-lateral axis. None of the patellae had embedded third body debris, but the embedded superior metal pin was exposed due to extreme damage in 4 patellas. The original femoral and tibial components were left in-situ in all knees at the time of revision, such that only the polyethylene tibial and patellar articulations were exchanged.

Discussion: Despite severe wear of the components, there was only a 5% incidence of osteolysis noted intra-operatively. Cyclic compressive and tensile forces during knee flexion likely caused initiation and propagation of cracks resulting in patellar bearing fracture. The delami-nation patterns on the retrieved patellae are consistent with bearing rotation into an incongruent bearing position during knee flexion, with presumably high contact stresses occurring in the delaminated superior-medial quadrant. Fully congruent mobile-bearing patella components must maintain mobility between the articular surface and metal back so that areas of incongruent contact, and associated high contact stresses and delamination, do not occur during in-vivo function.

Early revision after total knee arthroplasty (TKA) is fortunately uncommon. However, instability and lack of fixation are common early failure mechanisms. Cement techniques utilizing lavage and multiple drill hole interdigitation of the resected tibial surface can reduce micromotion and produce reliable tibial component fixation. This study looks at clinical failure mechanisms, cement technique and polyethylene damage in patients needing early revision of cemented TKA.

PCL-retaining TKA with cement fixation was performed on > 1000 patients at a single institution. Cement techniques varied with surgeon, with some using lavage and drill hole preparation of the resected surface and others electing to cement the surface “as cut”. Seventeen patients were revised within three years of follow-up. Revision reasons included loosening (41%), instability (18%), infection (24%), pain (12%), and malposition (6%). Prospective outcome scores, radiographic data, revision reasons, and polyethylene wear were compared.

Pre-revision pain and function scores gradually decreased back to pre-operative levels. Leg alignment averaged 7° varus (nine patients) and 12° valgus (eight patients) pre-operatively and 5° valgus at pre-revision. Tibial radiolucent lines were present medially only in nine knees and medially and laterally in four knees. The majority of patients revised for loosening had a tibial component cemented onto the “as cut” bone without additional preparation. Damage covered 32%-85% of the polyethylene articular surface. Scratching and pitting were significantly correlated (p< 0.05) with shorter in-situ time and revision for instability and loosening. Alignment and outcome scores were not correlated with damage.

In this series of cemented TKA, loosening and instability accounted for 59% of the early failure, similar to the incidence previously reported for cementless TKA. Cement technique and component positioning, not polyethylene wear, were the primary contributing factors. Attention to ligament balancing and achieving better tibial component fixation is needed to further limit the incidence of early failure after cemented TKA.

Both backside and articular surface wear have been linked to osteolysis after total knee arthroplasty (TKA). Prostheses with cementless fixation, screw holes in high load regions, and thin polyethylene are susceptible to backside wear. Factors associated with articular wear are similarly well defined. Micromotion at the modular polyethylene interface has been reported for many prostheses, but the relevance of such data compared to articular motions and wear are difficult to appreciate. This study compares in vivo motions and wear occurring at the backside and articular surfaces after TKA.

Contemporary PCL-retaining prostheses from one manufacturer were implanted by one surgeon using cement fixation. The polyethylene inserts were > 6mm thick with a full peripheral rim capture and anterior wire locking mechanism. Femoral condylar motions were measured in 20 knees using fluoroscopic analysis during stair and gait activities. All patients had good to excellent clinical outcomes at one year follow-up. Articular and backside surface damage was evaluated on 32 polyethylene inserts retrieved after 27 months (1 to 71) months in-situ for infection (9), autopsy (6), patellar resurfacing (4), patellar loosening (4), tibial loosening (3), osteolysis (2), and other (4).

Femoral condylar translation over the polyethylene articular surface ranged from 5-10 mm, which is substantially larger than the reported 50-500 micron range of backside interface micromotion measured in vitro. Damage covered < 33% of the backside surface and appeared as a cast impression of the opposed metal tibial component without scratches associated with micromotion. In contrast, damage consisting predominantly of scratching, burnishing and tractive striations covered 46% of the articular surface.

Different locking mechanisms for modular polyethylene inserts result in different degrees of backside wear. No significant backside wear was observed these retrieved inserts with a wire-supplemented peripheral capture. Given the abrasive wear mechanisms and particulate debris shed during femoral condylar sliding, efforts to control motions at the articular surface appear warranted.

How total knee replacements (TKR) articulate is directly related to their functional and wear performance. Recently, significant interest has concerned the center of axial rotation, or pivot point. Since the tibia exhibits internal rotation with knee flexion, the pivot point describes condylar translations: a medial pivot implies posterior lateral condylar translations with flexion, a lateral pivot implies anterior medial condylar translations with flexion.The purpose of this study was to describe the location of the pivot point, as related to TKR design, in a large number of knees studied under dynamic weight-bearing conditions.

Two hundred and four well functioning TKR’s were studied using fluoroscopy as subjects performed a stair ascent. There were 131 cruciate retaining fixed bearing knees (11 designs), 33 mobile bearing knees (5 designs), and 40 posterior stabilized knees (4 designs). CAD model based shape matching was used to determine 3D knee kinematics and the pivot point location from 21,837 images. The pivot location was described as a percentage of tibial width, −50% (lateral) to +50% (medial).

Posterior stabilized knees exhibited medial pivots (mean +14%, +7% to +30%) while cruciate retaining (mean −9%, −35% to +21%) and mobile bearing knees (mean −20%, −48% to +5%) exhibited lateral pivots on average (p< 0.001).

How a TKR design provides A/P stability dictates the location of its center of axial rotation and the A/P motions of the condyles. As the relationship between constraint and in vivo motions becomes clearer, TKR designs can be enhanced to achieve more favorable functional and wear performance.

Condylar liftoff can undoubtedly occur with total knee replacements (TKR); it occurs in the surgeon’s hands and has been shown to occur in vivo. However, the reported incidence of condylar liftoff and the implications for articular surface damage require further scrutiny. A three-part argument is made that the incidence of condylar liftoff has been overstated, and there is little direct evidence that condylar liftoff is a significant factor in the wear performance of coronally flat-on-flat TKR’s. First, an analysis of fluoroscopic measurement errors based on the uncertainty in measuring varus/valgus angles (the parameter used to determine liftoff) reveals that the standard error for liftoff measurements is 1.2mm, nearly identical to the mean liftoff value in recent published reports. Second, because most TKR’s have some anterior/posterior curvature of the tibial insert, any axial rotation of the knee induces a varus/valgus angulation that can be interpreted as liftoff, even though the condyles remain in contact. Third, condylar liftouff has been used to justify the need for coronally round-on-round geometries, yet an analysis of 100 unselected retrieved tibial inserts from three coronally flat designs reveals no difference in wear type, magnitude, severity, depth, or symmetry between the medial and lateral aspects of the tibial inserts. Although condylar liftoff certainly can occur in vivo, an argument can be made that the incidence of liftoff in experimental studies has been overstated, and that there is little evidence from retrievals that contemporary coronally flat-on-flat TKR’s are uniquely susceptible to articular damage from condylar liftoff.

The perception that all cemented stems have reasonable assurance of success if implanted with contemporary cement technique has recently been questioned. Surface finish, stem shape, patient weight and high neck offset have been identified as factors contributing to early loosening. Small design changes to existing cemented stems have led to substantial differences in clinical performance. This study investigates the multi-factorial nature of stem loosening after a 24% early failure rate occurred within the initial three years.

Total hip arthroplasty was performed by the same surgeon on 67 patients with an average age and weight of 65 years (21-85) and 82 kg (49-127), respectively. Initial diagnosis was osteoarthritis (84%), osteonecrosis (7%), congenital dysplasia (4%), and other (5%). Modern femoral stems (Perfecta IMC) with increased lateral neck offset were implanted using contemporary cementing techniques. The stems are grit-blasted proximally, with intramedullary collar steps on the anterior/posterior surface. These design features are meant to enhance axial load transfer and stem-cement bonding. All acetabular cups were uncemented and used polyethylene (48 patients) or cobalt-chrome (20 patients) liners. Patients were evaluated with clinical and radiographic follow-up.

Revision for stem loosening was necessary in 16(24%) patients 9 to 38 months after index surgery, including 9 hips revised within the first 18 months. Radiolucencies at the cement/bone interface, stem subsidence and distal femoral osteolysis were consistently observed. Patients with loosening were significantly heavier than those with well-fixed stems (93 kg versus 78 kg, respectively). Revised hips included 7 cobalt-chrome and 9 polyethylene articulations.

It appears that several mechanical factors contributed to these early failures. Lateral offset stems with cement fixation appear to be at risk for loosening in young, heavy patients. Design features and a thin cement mantle may have resulted in increased cement stresses and cracks during the axial and torsional loading that occur with daily activities.

Validation of input parameters and the resulting polyethylene damage is essential for knee joint wear simulators to be useful in prospective evaluations. The purpose of this study was to compare damage patterns on polyethylene inserts wear tested on a knee simulator with inserts retrieved after well-functioning total knee arthroplasty (TKA).

Five polyethylene tibial inserts from a PCL-retaining knee prosthesis (Natural Knee) were wear tested on an Instron/Stanmore simulator in 50% bovine serum. The input consisted of ISO 14243 force-controlled testing standard to simulate human gait for 5 million cycles. Nine polyethylene tibial inserts (Natural Knee) were retrieved from patients after 52+45 months (13-124) of successful function. The inserts were retrieved post-mortem (n=7) and for pain (n=2). Articular damage was assessed and the circumference of each damage region digitized. The damage size, location and linear surface deformation were measured and the deformation rate (mm/106 cycles or mm/year) was calculated.

The linear deformation rate for all inserts decreased considerably with time. Lateral damage was located significantly more posterior than the medial damage on both the simulated and retrieved inserts, corresponding to femoral external rotation. Retrieved inserts had larger lateral damage, whereas simulator inserts had larger medial damage. The AP extent of damage on the retrieved inserts was significantly greater than the extent on the simulator inserts. Three retrieved inserts had substantial delamination, whereas none of the simulator inserts had delamination.

There was good agreement in the deformation rates for the simulator and retrieved inserts. However, retrieved inserts with delamination show an increased deformation rate and this type of damage did not occur on simulator inserts. The greater AP extent of damage and larger lateral damage on the retrieved inserts suggest that in vitro wear simulation should perhaps include a more complete range of patient activity dynamics to better predict in vivo damage.

Aims: This study reports early outcome data from a contemporary tapered cementless stem. Methods: Fifty patients (55 hips) underwent THA and received cement-less femoral and acetabular components (Encore Medical, Austin, TX). All stems were collarless with a 3-dimensional wedge geometry manufactured from titanium alloy with a proximal full circumferential titanium porous coating. There were 19 females and 31 males with an average age and weight of 63(31–81) years and 86(46–114)kg, respectively. Initial diagnosis was osteoarthritis (76%), osteonecrosis (15%), congenital dysplasia (5%), rheumatoid arthritis (2%) and trauma (2%). Patients were followed prospectively and Harris Hip Scores were recorded. Pre-operative, immediate postoperative, and annual radiographs were evaluated for stem and cup position, component migration, and evidence of radiolucent lines and bony adaptation. Results: The average length of follow-up was 2.0±0.4(1.5–3.1) years. There were no cup or stem revisions at this early follow-up interval. The average Harris Hip Score was 95 points (range, 63 to 100). Average cup inclination was 53¡(38¡–65¡). Radiolucent lines around the acetabular cup were infrequent (7%) and were less than 1 mm in width. Stem subsidence was noted on 10 (18%) stems, consistent with the collarless stem geometry. Some medial calcar atrophy was noted in 78% of the hips. Six (11%) stems had distal radiolucent lines > 2 mm in width. One (2%) stem had pedestal formation at the distal stem. Conclusions: Excellent midterm outcomes have been reported for tapered cementless stems with proximal circumferential porous coating. Radiolucent lines in the distal stem zones and mild medial calcar atrophy have not been associated with inferior outcomes up to 10 years.

Aims: It has been suggested that the capture mechanism of modular polyethylene tibial inserts degrades with time in-situ. This study evaluates micromotion, polyethylene wear and tissue histology in contemporary cemented TKRñs retrieved at autopsy. Methods: Twelve cemented, PCL-retaining TKRñs of the same design were retrieved at autopsy after 41(15–74) months in-situ. Patient age and body weight averaged 73 years and 90 kg, respectively. Insert micromotion was measured according to published protocols on 6 of the 12 TKRñs in which the modular tibial component was undisturbed at retrieval and on 6 unused control components. Tissue histology was evaluated using a semi-quantitative grading system. Articular and backside surface damage was measured using published techniques. Results: There was no signiþcant difference (t-test, p=0.12) between the micro-motion index for retrieved inserts (154±121 um) and control inserts (62±53 um). Backside surface damage covered 38%±23% and was dimpled in appearance without scratching or pitting. Damage covered 46%±8% of the articular surface. Micromotion was negatively correlated with in-situ time (r=−0.94) and backside damage (r=− 0.97). Conclusions: Micromotion for these autopsy-retrieved TKRñs is less than half the 380 micron magnitude measured for other autopsy-retrieved designs, as reported by Engh. Micromotion was greatest on inserts with the least backside wear and the shortest time in-situ. These data suggest that backside damage resulted from axial compression of the polyethylene insert against the baseplate rather than micromotion.

Aims: During total knee replacement (TKR), bone dimensions are sometimes between implant sizes. Many surgeons select the smaller tibial component size to avoid overhanging the cortex and the smaller femoral component size to avoid overstuffing the joint space. However, the larger femoral component size is sometimes selected to ensure adequate bone coverage. This study was initiated after contact between the polyethylene insert and posterior femoral osteophytes was observed at autopsy even though radiographs showed adequate osteophyte removal at index TKR. It was hypothesized that knees implanted with the same sized femoral and tibial components would have a higher incidence of contact and abrasive wear than knees with larger femoral components. Methods: Thirty five polyethylene inserts were retrieved at autopsy (34%) and at revision TKR for reasons of infection (20%), patellar complications (17%), loosening (11%), patellar resurfacing (14%), and supracondylar fracture (3%). Mean implantation time was 32(1–74) months. Articular damage was evaluated using light microscopy. Results: Nineteen knees had smaller femoral components and 13 (68%) had abrasive wear on the polyethylene rim. Sixteen knees had larger femoral components and 4 (25%) had the abrasive wear pattern. The incidence of abrasive wear was significantly lower in knees with larger sized femoral components (Fisher’s Exact, p< 0.05). Conclusions: Surgeons are encouraged to remove all osteophytes that might impinge and avoid sizing the components such that the polyethylene insert overhangs the femoral component. Selecting the larger femoral component size may lessen impingement and provide some benefit for increasing knee flexion and decreasing abrasive wear.

Aims: This study describes the varied anatomic geometry of the proximal femur as it relates to cementless femoral stem design. Methods: Femoral head offset, neck-shaft angle, and canal width were measured on frontal plane radiographs of undiseased hips from 47 female and 39 male North American patients. Average age and height was 66(21–89) years and 170(152- 193)cm, respectively. Patients were positioned supine with inward pointed toes to achieve femoral internal rotation during radiography. Measurements were corrected for magniþcation and correlated with demographic data. Results: The average femoral head offset and neck-shaft angle was 37±6(25- 53)mm and 131û±6û(119û-145û), respectively. Distal canal width aver aged 11±2(5–16)mm and the canal ßare index was normal in 77% of the femurs, stovepipe in 8% and ßuted in 15%. Signiþcant correlations were as follows. As patient height increased, femoral head offset and distal canal width also increased (p< 0.05). As patient age increased, distal canal width increased and canal ßare index decreased (p< 0.05), becoming more stovepipe in shape. Femoral head offset decreased with increased neck shaft angle. Conclusions: Comparing these data with published data from Eastern populations is useful for determining appropriate cementless stem sizes applicable to different populations. Variation in femoral head offset supports the use of different neck lengths, neck-shaft angles or medialized necks to provide surgeons with stems suitable for restoring leg length and balancing soft tissues. The width of the distal endosteal canal varied widely, suggesting that some patients may beneþt from a smaller or larger distal stem diameter for a given proximal stem size.