Influence of Hip Position on Knee Flexion Angle in Patients Undergoing Total Knee Arthroplasty

The Journal of Arthroplasty Vol. 20 No. 5 2005 Influence of Hip Position on Knee Flexion Angle in Patients Undergoing Total Knee Arthroplasty Tomoyuki Matsumoto, MD,*y Nobuhiro Tsumura, MD,* Seiji Kubo, MD,y Ryoichi Shiba, MD,* Masahiro Kurosaka, MD,y and Shinichi Yoshiya, MDy Abstract: The influence of hip position on knee flexion angle before total knee arthroplasty (TKA) and that after TKA were investigated and compared. Sixty-six patients (70 knees) with osteoarthritis who were undergoing TKA were examined using lateral radiographs of maximum knee flexion angle with the hip joint at 08 extension and 908 flexion. The postoperative rate of decrease in knee flexion angle caused by hip extension was significantly larger compared with the value before surgery. The preoperative rate of decrease in knee flexion angle caused by hip extension showed strong inverse correlation with the preoperative and postoperative knee flexion angle ratio. In conclusion, tightness of the extensor mechanism is present in all knees undergoing TKA and especially has a strong influence on the postoperative flexion angle of the knee. Key words: total knee arthroplasty, tightness of the extensor mechanism, knee flexion angle, osteoarthritis, hip position. n 2005 Elsevier Inc. All rights reserved. One of the most important goals of total knee arthroplasty (TKA) is to improve the functional range of flexion to a minimum of 908, which is required for normal daily activities. Many factors influencing the amount of flexion after TKA have been investigated. Ritter and Stringer [1] evaluated the predictive value of patient age, sex, diagnosis, and preoperative motion after TKA and found that postoperative flexion may be predicted on the basis of the amount of preoperative flexion and that prosthetic design, sex, age, and diagnosis of rheumatoid arthritis did not alter this correlation. Several others [2-15] have subsequently recognized the importance of preoperative motion for postoperative results. From the *Department of Orthopedic Surgery, Hyogo Rehabilitation Center Hospital, Kobe, Japan, and ydepartment of Orthopedic Surgery, Kobe University Graduate School of Medicine, Kobe, Japan. Submitted December 26, 2003; accepted October 24, 2004. No benefits or funds were received in support of the study. Reprint requests: Tomoyuki Matsumoto, MD, Department of Orthopedic Surgery, Kobe University Graduate School of Medicine, 7-5-2 Kusunoki-cho, Chuo-ku, Kobe 650-0017, Japan. n 2005 Elsevier Inc. All rights reserved. 0883-5403/05/1906-0004$30.00/0 doi:10.1016/j.arth.2005.01.011 Factors influencing range of flexion after TKA are mainly classified into 2 factors: intracapsular and extracapsular. Intracapsular factors, including implant design, ligament balancing, flexion-extension gap balance, height of joint line, and patella resurfacing, have been discussed by many authors [3,5,7,13,14,16]. However, extracapsular factors such as stiffness of the quadriceps or tightness of the extensor mechanism have received little attention. Therefore, the purpose of this study was to evaluate the influence of the tightness of the extensor mechanism on the range of knee flexion in patients undergoing TKA by comparing the flexion angle with the hip in extended and flexed positions. Materials and Methods One hundred forty primary TKAs were performed at the Hyogo Rehabilitation Center Hospital from May 2002 to May 2003. During that period, flexion angles were measured under fluoroscopic control preoperatively and at a mean of 2 months postoperatively. To make a fair assessment and minimize the influences of clinical 669

670 The Journal of Arthroplasty Vol. 20 No. 5 August 2005 Table 1. Parameters Examined 1: Decrease in knee flexion angle caused by hip extension ¼ maximum knee flexion ð908 hip flexion Þ maximum knee flexion ð08 hip extensionþ 2: Rate of decrease in knee flexion angle caused by hip extension ¼ decrease in knee flexion angle caused by hip extension maximum knee flexion ð908 hip flexionþ 3: Preoperative and post operative knee flexion angle ratio ðpost operative value=pre operative valueþ ¼ postoperative maximum knee flexion angle ð 908 hip flexion Þ preoperative maximum knee flexion angle ð908 hip flexionþ variables, cases of this study were indicated for the subjects without valgus deformity, severe bony defects, and rheumatoid arthritis. Knees with posterior-stabilized TKA were excluded. The remaining 66 patients diagnosed with osteoarthritis constituted the study. Sixty patients (63 knees) were women and 6 (7 knees) were men (age range, 45 90 years; mean, 70.2 F 7.6 years). Surgeries were performed by the senior author (NT) using posterior cruciate ligament retaining TKA (PFC Sigma, DePuy Inc, Warsaw, Ind) with a standardized surgical technique. The knees were exposed with a medial parapatellar arthrotomy and the bony resection was carried out using the measured resection technique with intramedullary alignment rods for the femur and extramedullary alignment rods for the tibia. Radiographic Measurements Lateral radiographs were taken at the maximum knee flexion of 2 different hip positions under fluoroscopic control by the same physician, one at 08 extension of the hip joint and the other at 908 flexion of the hip joint. The patients were set in the lateral decubitus position on the image intensifier. The objective knee was in contact with the table and the opposite knee was on an adequate box to stabilize the leg and pelvis. In the hip-extended position, the contralateral knee and hip were in a 908 flexed position with the knee fixed on the box. In the hip-flexed position, the contralateral knee and hip were in a 08 extended position with the knee fixed on the box. The x-ray measurements were performed preoperatively and at 2 months postoperatively. On the lateral views obtained at the 2 periods, the knee angle was determined by a physician who was blind to the measuring method by measuring between the axis of the distal femur and that of the proximal fibula. Examined Parameters We defined 3 parameters as follows (Table 1): 1. Decrease in knee flexion angle caused by hip extension was defined as maximum knee flexion at 08 hip extension subtracted from that at 908 hip flexion. Table 2. Difference Between Maximum Knee Flexion Angle at 08 Hip Extension and at 908 Hip Flexion and Comparison of Preoperative and Postoperative Rates of Decrease in Knee Flexion Angle Caused by Hip Extension Maximum knee flexion angle 908 hip flexion (%) 08 hip extension (%) Rate of decrease in knee flexion angle caused by hip extension Preoperative 128.98 F 17.38 121.58 F 14.28* 5.468 F 3.468 Postoperative 117.48 F 14.18 108.18 F 14.28* 7.888 F 4.778y Values are presented as mean F SD. This table describes the Student t test results of the preoperative and postoperative maximum knee flexion caused by hip position and shows the preoperative and postoperative rate of decrease in knee flexion angle caused by hip extension. A statistically significant decrease from 908 hip flexion to 08 hip extension was demonstrated both before and after TKA. Rate of decrease in knee flexion angle caused by hip extension showed significant increase postoperatively. *Statistical difference between hip flexion and extension ( P b.01 vs hip flexion). ystatistical difference between the preoperative and postoperative rate ( P b.01 vs the preoperative rate).

Influence of Hip Position! Matsumoto et al 671 2. Rate of decrease in knee flexion angle caused by hip extension was defined as decrease in knee flexion angle caused by hip extension divided by maximum knee flexion at 908 hip flexion. 3. The preoperative and postoperative knee flexion angle ratio (postoperative value/preoperative value) was defined as the postoperative knee flexion divided by the preoperative knee flexion. Statistical Analysis Statistical analysis was performed using a statistical software package (Statview 5.0, Abacus Concepts Inc, Berkeley, Calif). A P value of less than.05 was considered significant. Paired Student t test and linear regression were used. The difference between maximum knee flexion angle at 908 hip flexion and that at 08 hip extension was analyzed using the paired Student t test. Comparison of preoperative and postoperative rates of decrease in knee flexion angle caused by hip extension was made using the paired Student t test. Correlation between the preoperative rate of decrease in knee flexion angle caused by hip extension and the preoperative and postoperative knee flexion angle ratio was analyzed using linear regression. Pre and post-operative knee flexion angle ratio (%) 130 120 110 100 90 80 70 60-4 -2 0 2 4 6 8 10 12 14 Rate of decrease in knee flexion angle caused by hip extension (%) Y= 102.264-1.925 * X; R= -0.709, R 2 = 0.502, p<0.0001 Fig. 1. Correlation between the rate of decrease in knee flexion angle caused by hip extension and the preoperative and postoperative knee flexion angle ratio. This figure shows regression analysis results. The slope is interpreted as the extent to which the rate of decrease in knee flexion angle caused by hip extension affects the preoperative and postoperative knee flexion angle ratio. R 2 is the measure of the predictive value of the regression coefficients. Results Difference Between Maximum Knee Flexion Angle at 908 Hip Flexion and that at 08 Hip Extension Preoperative maximum knee flexion angles at 908 hip flexion and 08 hip extension were 128.98 F 17.38 and 121.58 F 14.28, respectively. The difference was statistically significant ( P b.0001; Table 2) as the maximum knee flexion angle of 7.48 decreased at 08 hip extension. Postoperative maximum knee flexion angles at 908 hip flexion and 08 hip extension were 117.48 F 14.18 and 108.18 F 14.28, respectively. The difference was statistically significant ( P b.0001; Table 2) as the maximum knee flexion angle of 9.38 decreased at 08 hip extension. Comparison of Preoperative and Postoperative Rates of Decrease in Knee Flexion Angle Caused by Hip Extension Preoperative and postoperative rates of decrease in knee flexion angle caused by hip extension were 5.46% F 3.46% and 7.88% F 4.77%, respectively, with a statistically significant difference ( P =.0007; Table 2). Correlation Between Preoperative Rate of Decrease in Knee Flexion Angle Caused by Hip Extension and Preoperative and Postoperative Knee Flexion Angle Ratio The preoperative rate of decrease in knee flexion angle caused by hip extension (R = 0.709; P b.0001) showed inverse correlation with the preoperative and postoperative knee flexion angle ratio (Fig. 1). Discussion The extensor mechanism of the knee, which mainly consists of quadriceps mechanism, patella, and patellar tendon, plays a vital role in TKA. Hsu et al [17] pointed out that increasing patellar thickness might reduce the range of motion of the knee because of increasing stress in the extensor mechanism and might predispose to patellar subluxation. Johanson [18] described that restricted range of motion was frequently responsible for intraoperative avulsion of the patellar tendon because of the inability to flex the knee beyond 908, with associated scarring in the lateral gutter producing excessive tension on the patellar tendon during the surgery. King et al [19] found that the

672 The Journal of Arthroplasty Vol. 20 No. 5 August 2005 skin over the knee stretched 40% between extension and flexion, and it followed that the entire extensor mechanism underwent a similar lengthening with range of motion. Emerson et al [20] suggested that closing the extensor mechanism in flexion may produce a measurable improvement in knee flexion over that achieved by an extension closure. All these studies have suggested the importance of the extensor mechanism on postoperative knee flexion angle. Besides, Rothstein et al [21] showed the importance of consistent positioning of the hip when measuring knee motion, especially during knee flexion. Based on these previous findings, this study was undertaken to evaluate how the range of postoperative flexion is affected by the amount of tightness of the extensor mechanism by comparing the knee flexion angles with different hip positions. Consequently, several significant findings were drawn from the analysis of the measurements. First, the present study showed a significant decrease in maximum knee flexion angle by hip extension both preoperatively and postoperatively, suggesting that hip position was considered one of the most important factors determining maximum knee flexion angle. Therefore, to measure and assess the accurate knee flexion angle, standardizing hip position is important when measuring knee flexion. Moreover, the results may indicate that tightening in the extensor mechanism exists both preoperatively and postoperatively. We hypothesized the mechanism of decrease in maximum knee flexion angle by hip extension as follows. When the hip joint is extended, the rectus femoris muscle is stretched and tightness of the extensor mechanism increases, leading to a corresponding decrease in maximum knee flexion angle in the hip-extended position in patients with osteoarthritis (Fig. 2). Second, the decrease in knee flexion angle caused by hip extension was observed to a larger degree postoperatively than before surgery, suggesting that the influence of tightening in the extensor mechanism might increase postoperatively as a result of the surgical damage at TKA. Krackow [22] described that the tight suturing of the capsule and the tendinous layer might cause an extensor tenodesis effect, leading to the decreased extensibility of the extensor mechanism. In addition, he described that the common state of the knee joint capsule and subcapsular and intracapsular synovial-like scars after TKA could predict some form of immobilization and protection, producing a tightening of the structures. As in his description, our study might indicate that the scarring tissue caused tightening or shortening of Fig. 2. Relationship between hip position and knee flexion angle. A, In the hip-flexed position, a loose extensor mechanism causes the knee flexion angle to increase. B, In the hip-extended position, a tight extensor mechanism causes the knee flexion angle to decrease. the extensor mechanism, resulting in restricted postoperative flexion. Therefore, to maintain the amount of flexion after TKA, surgeons should pay attention to the tension and mobility of the extensor mechanism. Finally, the present study showed an inverse correlation between the preoperative rate of decrease in knee flexion angle caused by hip extension and the preoperative and postoperative knee flexion angle ratio. In other words, patients with significant preoperative loss of knee flexion caused by hip extension tended to lose more flexion, whereas patients with low loss of knee flexion caused by hip extension tended to gain better flexion. Many authors [1-15] have subsequently recognized the importance of preoperative motion for postoperative results. However, the present study showed that patients who had the equal preoperative knee flexion angle had the various flexion angles postoperatively because of the amount of loss of knee flexion caused by hip extension. Moreover, the present study showed

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