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.

Many total knee replacements (TKR) are designed with more conforming articular geometry to increase the femoral contact area and decrease surface stresses. These designs are supported by studies suggesting that implants with coronally flat articular surfaces are vulnerable to medial-lateral lift-off and edge-loading on the polyethylene insert. However, few retrieved inserts from contemporary TKR’s have shown wear consistent with this loading mechanism. This study presents wear measurements from 37 consecutively retrieved polyethylene inserts of the same PCL-retaining design with coronally flat-on-flat articulations. If substantial edge-loading occurred in-vivo, it was hypothesise that wear would be located closer to the medial or lateral edge of the articular surface with a high incidence of delamination.

Inserts were retrieved at autopsy (n=12) after 41 (15–74) months in-situ and at revision TKR (n=25) after 26 (1–71) months in-situ. Reason for revision was infection (28%), patellar component complications (24%), loosening (24%), patellar resurfacing (20%), and supra-condylar fracture (4%). Articular damage was measured using light microscopy and digitising the circumference of each damage region on calibrated images. Surface deformation was measured relative to unused control inserts using a hand-held digitising stylus.

Wear patterns were not significantly different between autopsy or revision retrievals (ANOVA, p> 0.05). Articular wear covered 48%+16% and 47%+14% of the medial and lateral surfaces, respectively. The most frequent wear modes were burnishing and scratching. Delamination occurred on 4(11%) inserts, but involved < 2% of the articular surface. Wear patterns were internally rotated and centrally located. Not one insert had a wear area centroid located in the medial or lateral third of the articular surface. Surface deformations were greatest in the inserts’ central region and the linear deformation rate decreased with time.

Concerns of high contact stresses associated with edge-loading were unsupported by these retrievals. Condylar lift-off, if it occurs, does not appear to substantially impact polyethylene damage in coronally flat-on-flat articulations.