MATERIALS AND METHODS
Three different total knee designs were evaluated. The designs consisted of a flat, a curved, and a mobile bearing tibial inlay. In most designs, the tibial radius remains constant, whereas the femoral radius in the sagittal plane is dependent on the knee flexion angle. The femoral radius for the flat and the curved design was 25 mm for knee flexion angles from 0 to 60°. From 60 to 110°, the radius decreased to 16.5 mm. The mobile bearing design showed a femoral radius of 37 mm from 0 to 21°. From 21 to 86°, the radius changed to 19 mm, and above 86° the radius decreased further to 17 mm. For all three designs, a medium-sized prosthesis was used. The polyethylene thickness of the tested inlays was 11 mm for the flat design, 9.4 mm for the curved design, and 11.5 mm for the mobile bearing design. It has been shown that the thickness of the polyethylene has a minimal effect on the surface and von Mises stress if it is greater than 6–8 mm (4). Each design was tested with a load of 4 BW (2904 N) near full extension (e.g., power walking) (14,20). Because the radius of the femoral components did not change between 40 and 60° for all three designs, it was possible to test the designs for 8 BW (5808 N) and 9 BW (6533 N) at about 40–50° of simulated knee flexion (e.g., downhill walking (14) and jogging (27)). For cycling, the designs were tested with a load of 1.2 BW at 80° of knee flexion (12). All designs were cemented into a metal block and then mounted on a material testing system (Zwick, Materialprüfmaschine, Ulm, Germany). The tibial plateau and the femoral component were preloaded with 50 N with all adjusting screws loose. The femoral component could adjust itself at the lowest point of the tibial inlay, and an even load distribution was ensured between the medial and lateral compartment. The screws were then tightened and the femoral component was lifted off the inlay to insert the Fuji prescale film. Low prescale film was used for the mobile bearing design near full extension. Medium prescale films were used for all other measurements. The load was increased with 100 N·s−1. The final load was applied for 3 s and then decreased again with 100 N·s−1. For each load, five measurements were obtained. The films were scanned (Hewlett-Packard Scanjet 4c, Avondale, PA) and an image analysis program (Image Pro, Media Cybernetics, Silver Spring, MD) was used to obtain the total contact area and overloaded area. The latter was defined as the area, which recorded stress levels above the yield point of ultrahigh molecular weight polyethylene (UHMWPE). This parameter has been suggested for the assessment of implant designs rather than peak stress values (7). The yield strength is the nominal stress at yielding, or the change from elastic to measurable plastic deformation. In many materials, it is difficult to pin point the yield point on the stress-strain curve (2). Hence, the yield strengths of UHMWPE has been recorded from 12.7 MPa (9) to 32 MPa (8). This study assumed a yield point of 25 MPa (2,10).
The contact area increased with increasing loads for all three designs due to indentation of the femoral condyle in the tibial inlay. The overall contact area was greater during power walking, downhill walking, or jogging for the mobile bearing design when compared with the flat or curved designs. The highest contact area was found for the mobile bearing design during power walking because this design showed full conformity between 0 and 20° of knee flexion (Fig. 1).
The overloaded area (area with stress levels above 25 MPa) for each design during cycling, power walking, downhill walking, and jogging are reported in Figure 2. A small overloaded area ranging from 7.6 to 13.6 mm2 occurred during cycling for all three designs. During power walking, the mobile bearing design showed no overloaded area. For the flat and curved designs, the overloaded area remained well below 50 mm2. During downhill walking and jogging, the overloaded area reached levels as high as 180 mm2. Hence, 40–70% of the total contact area was overloaded during downhill walking or jogging.
There are few studies addressing sports involvement after total hip or total knee replacement. Dubs et al. (11) retrospectively analyzed the charts of 150 younger patients over an average follow-up period of 5.8 yr. They found no increased loosening of total hip replacements in patients with intense sporting activities and concluded that there was no need to prohibit sport after total hip replacement. This was in contrast to Kilgus et al. (13), who found a two-fold increased risk of revision surgery after total hip replacement for patients who participated regularly in sporting activities or heavy labor. However, the effects of patients activities were not seen until 10 yr postsurgery.
McGrory et al. (18) interviewed 28 orthopaedic surgeons about their recommendations for sports after total hip or total knee replacement. If 75% of the surgeons considered the activity safe, it was labeled “recommended”; and if 75% did not allow their patients to participate in a certain activity, it was identified as “not recommended.” The same activities were recommended by these surgeons after total hip or total knee replacement (18). We believe that it is important to distinguish between suitable activities after total hip or total knee replacement because of the different geometries. Although total hip replacements are designed as a ball and socket joint with full congruency throughout the flexion range, many total knee replacements exhibit a mismatch between the femoral and tibial radius with high peak pressures on the polyethylene inlay. These high pressures are above the yield point and could lead to increased wear including delamination and destruction of the inlay. Mallon and Callaghan found a higher incidence of pain and radiographic lucent lines in active golfers after total knee replacement (16) than after total hip arthroplasties (15). The authors are also aware of one patient who participated in a marathon run after total knee replacement despite being discouraged by the surgeon. The polyethylene inlay broke at the 35-km mark due to severe delamination and destruction. In order to recommend suitable physical activities after total knee replacement, it is important to consider both the load and the knee flexion angle of the peak load. Many designs show increased conformity near full extension but the conformity ratio decreases beyond 30° due to the reduced femoral radius. Activities such as hiking or jogging may pose two main problems for a modern knee design. First, the loads can reach up to 8 or 9 BW. Second, the peak loads occur between 40 and 60° of knee flexion, where many knee designs do not display a high conformity. The present study showed that 40–70% of the overall contact area was stressed above the yield point when applying tibiofemoral loads that occur during jogging or downhill walking. Also, the overloaded contact area reached levels of more than 150 mm2. Regular jogging or hiking with intense downhill walking produces a large overloaded area that may endanger the polyethylene inlay of most current total knee prostheses.
During power walking, the tibiofemoral load can reach up to 4 BW at 20° of knee flexion (14,20). At 20° of knee flexion, the tested mobile bearing design was nearly conforming, and the peak stress was never above the yield point of polyethylene. For the flat and curved inlay, power walking produced some stress levels above the yield point. However, the overloaded area was three times smaller than that found for jogging or downhill walking. Therefore, it is suggested that power walking can still be permitted after a total knee replacement.
The peak load for cycling was found at 80° of knee flexion (12). During these angles of flexion, most designs show little conformity. However, the tibiofemoral load is as small as 1.2 BW, which reduces the stress levels on the inlay. The present study showed that the overloaded area for 1.2 BW was extremely small ranging between 7.6 and 13.6 mm2. It was concluded that cycling can be performed after total knee replacement. To further reduce forces about the knee when cycling, patients should place the bicycle seat as high as possible (12).
Some limitations of the present study must be mentioned. First, only static loads were applied. However, the tested loads represent the peak values for these activities. Hence, the additional values of a dynamic investigation would be smaller and not alter the conclusions. Second, an even load distribution was assumed. Because it is known that high adduction moments may occur (17), the present study may underestimate the stress on the polyethylene. Third, torques and rotatory stress occurring during these activities were not considered in the present study. However, until more sophisticated biomechanical investigations are developed, the following recommendations are proposed.
Cycling and power walking seem to be the least demanding endurance activities for the knee joint. Patients performing endurance sports after total knee replacement should alternate activities such as power-walking and cycling. Because cycling and power walking load the knee joint at different flexion angles, different parts of the tibial inlay will be stressed and ensure a more even wear pattern. For mountain hiking, patients are advised to avoid descents or at least use ski poles and walk slowly downhill to reduce the load on the knee joint. Furthermore, they should not carry a heavy backpack. Regular jogging, or sports involving running, should be discouraged after total knee replacement.
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