Stephen J. Incavo

The transepicondylar axis approximates the optimal flexion axis of the knee

The traditional understanding of knee kinematics holds that no single fixed axis of rotation exists in the knee. In contrast, a recent hypothesis suggests that knee kinematics are better described simply as two simultaneous rotations occurring about fixed axes. Knee flexion and extension occurs about an optimal flexion axis fixed in the femur, whereas tibial internal and external rotations occur about a longitudinal rotation axis fixed in the tibia. No other translations or rotations exist. This hypothesis has been tested. Tibiofemoral kinematics were measured for 15 cadaveric knees undergoing a realistic loadbearing activity (simulated squatting). An optimizing technique was used to identify the locations of the optimal flexion and longitudinal rotation axes such that simultaneous rotations about them could best represent the measured kinematics. The optimal flexion axis was compared with the transepicondylar axis defined by bony landmarks. The longitudinal rotation axis was found to pass through the medial joint compartment. The optimal flexion axis passed through the centers of the posterior femoral condyles. No significant difference was found between the optimal flexion and transepicondylar axes. To an average accuracy of better than 3.4 mm in translation, and 2.9° in orientation, knee kinematics were represented successfully by simple rotations about the optimal flexion and longitudinal rotation axes. The optimal flexion axis is fixed in the femur and can be considered the true flexion axis of the knee. The transepicondylar axis axis, which is identified easily by palpation, closely approximates the optimal flexion axis.

Clinical performance of a highly cross-linked polyethylene at two years in total hip arthroplasty: a randomized prospective trial

We report the 2-year results for a prospective randomized trial comparing highly cross-linked with standard polyethylene in total hip replacements. In our study, 46 hips were available for radiographic analysis at 2- and 3-year follow-up. Femoral bearings were 28-mm cobalt chrome with the polyethylene insert randomly selected at the time of implantation to be highly cross-linked polyethylene (Crossfire; Stryker Howmedica Osteonics, Allendale, NJ) or standard polyethylene (N(2)/Vac, Stryker Howmedica Osteonics). Polyethylene wear rates were measured based on anterior-posterior (AP) and lateral pelvis radiographs at 6 weeks and at yearly intervals using a validated computer-assisted edge-detection method. Wear rates between the 2 groups were compared using the nonparametric Mann-Whitney test at the 95% level. A significant reduction in 2- and 3-dimensional linear wear rates (42% and 50%) was found in the highly cross-linked group (P =.001 and P =.005).

Tibial Axis and Patellar Position Relative to the Femoral Epicondylar Axis during Squatting

A laboratory-based study was performed to describe the tibial axis and patellar position relative to the femoral epicondylar (FE) axis during squatting. During the squat, the angle between the tibial and FE axes averaged 90.5 degrees, and 66% of internal rotation of the tibia occurred before 15 degrees flexion. In the mid-sagittal plane of the femur, the patella followed a circular arc, and mediolateral patellar shift averaged 4.3 mm. These findings can be used as the basis for development of new total knee arthroplasty components that recreate normal patellofemoral kinematics, and may provide important guidelines for alignment of the tibial and femoral components. The perpendicular relationship between the tibial and the FE axes may be useful in locating the FE axis intraoperatively. The reduced mediolateral shift of the patella suggests that alignment of the femoral component with the FE axis will aid patellar tracking about a circular arc with small deviations in the medial-lateral direction.

Early revision for component malrotation in total knee arthroplasty

Component malrotation may result in unsuccessful total knee arthroplasty. We asked whether revision improves function in patients with malrotated total knee arthroplasty components. We retrospectively reviewed 22 revision total knee arthroplasties performed for femoral and/or tibial component malrotation. Revision surgery was performed within 2 years of the primary arthroplasty in 81% of the cases (18 of 22) with the remainder within 5 years. Although all patients had pain, 32% of patients had associated instability and 36% of patients had poor range of motion. Average Knee Society Scores improved from 42 preoperatively to 77 postoperatively. Average Oxford Knee Scores improved from 38 preoperatively to 29 postoperatively. Although clini cal and functional improvement was observed, these results are inferior to those for primary knee arthroplasty, and they emphasize the need for proper component rotational positioning during primary total knee arthroplasty. Internal component malrotation leads not only to patellofemoral problems, but also to difficulty in gap balancing and femoral component sizing, which may in turn lead to either poor range of motion or symptoms of knee instability.Level of Evidence: Level IV, diagnostic study. See the Guide lines for Authors for a complete description of levels of evidence.

Anatomic rotational relationships of the proximal tibia, distal femur, and patella: Implications for rotational alignment in total knee arthroplasty

The orientation of the femur, tibia, and patella are important considerations in total knee arthroplasty. Our goal was to describe the relationships between the femoral epicondylar (FE) axis, posterior femoral (PF) axis, posterior tibial (PT) axis, patellar (PAT) axis, and patellar ligament (PL). A secondary goal was to determine where the short axis of the tibial tray intersects the patellar ligament as a function of tibial component rotation. Thirty normal magnetic resonance imaging (MRI) scans were analyzed. Strong relationships were found between the FE and PAT axes (2 degrees +/- 3 degrees, r(2) = 0.73), and between the FE and PF axes (6 degrees +/- 2 degrees, r(2) = 0.77). When the tibial baseplate was aligned along the PT axis, 30% of the cases were in an ideal position. When the FE axis was used, 73% were ideal.

Femoral component revision using an extensively hydroxyapatite-coated stem☆

Femoral component revisions with extensively coated stems have shown promising clinical results, although concerns over stress shielding still exist. We retrospectively reviewed 59 patients undergoing femoral component revision with an extensively hydroxyapatite (HA)-coated stem. The average length of follow-up was 3.3 years (range, 2-5 years). The average preoperative Harris Hip Score was 43 points, which improved to 86 points at the latest follow-up (P < .01). The overall mechanical failure rate was 2%. No evidence of stress shielding was seen in 78% of patients. The clinical results of this series using an extensively HA-coated stem are similar to those using an extensively porous-coated stem. Long-term follow-up is required to determine if an extensively HA-coated implant will be superior to an extensively porous-coated implant with regard to stress shielding.

Femoral stem insertion generates high bone cement pressurization.

Adequate bone cement pressurization is critical in obtaining optimal femoral cement mantles during total hip arthroplasty. Pressurization can be generated during insertion of the femoral stem into the cement-filled canal. This may be clinically useful in augmenting conventional cement gun pressurization. Two factors, which were expected to influence the amount of insertion-induced pressurization, are the cement’s cure state (viscosity) at the time of insertion and the femoral stem profile. This study evaluated the effect of these factors on cement pressurization during stem insertion. Femoral stems were inserted at a controlled rate into a reusable, simulated femoral canal. Intramedullary pressures were monitored at four locations along the canal’s medial midline. The intrusion factor quantity, which accounts for pressure magnitude, duration of pressurization, and cement viscosity, was developed to quantify pressurization. Stem insertion into late cure stage (high viscosity) cement resulted in significantly higher intramedullary pressures (as much as 187% higher) and intrusion factors (as much as 43% higher) as compared with early stage (low viscosity) cement. The highest pressures and intrusion factors were found in the distal canal. A tapered stem profile resulted in significantly higher pressures (as much as 65%) and higher intrusion factors (as much as 63%) than a straight stem.

Longitudinal crack propagation in bone around femoral prosthesis

Intraoperative femoral fracture is a well-recognized technical complication of cementless total hip arthroplasty. This study was designed to establish an in vitro model for initiation and propagation of fractures of the proximal femur in cementless THA and to assess the effect of fracture fixation in the form of cerclage wiring and drilling a hole in the distal extent of the fracture line. Fourteen human anatomic femur specimens were studied. Longitudinal cracks were made and propagation was performed on a materials testing system machine. A drill hole at the tip of a longitudinal crack does not prevent crack propagation. However, cerclage wiring has a statistically significant effect (p less than 0.025) on the ability of the fractured femur to withstand increased load.

Cementless hemispheric acetabular components: A 4- to 8-year follow-up report

A retrospective radiographic analysis was performed on 71 cementless acetabular components followed from 4 to 8 years after surgery. A Harris-Galante (Zimmer, Warsaw, IN) cup was used in 46 cases and an Optifix (Smith & Nephew Richards, Memphis, TN) cup was used in 25 cases. There were 66 primary and 5 revision cases. Nonprogressive radiolucencies were common in both types of cups (64% of Optifix, 70% of Harris-Galante). Radiolucencies were almost exclusively less than or equal to 1 mm in width and were most common in zones 1 and 3. Ten cups had continuous but nonprogressive radiolucencies, none greater than 1 mm in all three zones. No radiolucent lines wider than 2 mm were seen in any case. Four cups had progressive radiolucency that stabilized. Radiolucency around fixation screws was seen in one case, and demonstrated stable ingrowth at revision surgery. No cases of osteolysis, screw breakage, migration, or loss of fixation surface occurred. A single case of a broken locking mechanism of a Harris-Galante cup 2 years after liner exchange is reported.

Preformed Articulating Knee Spacers in 2-Stage Total Knee Revision Arthroplasty

Two-stage revision arthroplasty using articulating spacers for the treatment of infected total knee arthroplasty (TKA) is a successful management technique. Our purpose was to report our results using preformed, commercially available articulating spacers made of gentamicin-impregnated cement. Thirty-three patients with infected primary or revision TKAs were treated with these spacers using a 2-stage revision technique. In most cases, the spacers were modified intraoperatively by adding a stem of reinforced antibiotic-impregnated acrylic cement. Successful eradication was achieved in 30 of 33 cases at a minimum 2-year follow-up interval. Two patients required a second spacer before successful revision TKA. No spacer fractures or dislocations occurred in this series. No adverse soft tissue effects were noted from the use of this type of articulating spacer.