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2 Clinical Biomechanics 24 (2009) Contents lists available at ScienceDirect Clinical Biomechanics journal homepage: Gait analysis after bi-compartmental knee replacement He Wang a, *, Eric Dugan a, Jeff Frame a, Lindsey Rolston b a Biomechanics Laboratory, School of Physical Education, Sport, and Exercise Science, Ball State University Muncie, IN 47306, USA b Henry County Center for Orthopedics, New Castle, IN 47362, USA article info abstract Article history: Received 20 January 2009 Accepted 21 July 2009 Keywords: Knee OA BKR Gait Knee mechanics Background: It is reported that a majority of the patients with knee osteoarthritis have cartilage degeneration in medial and patellofemoral compartments. A bi-compartmental knee replacement system was designed to treat osteoarthritis at medial and patellofemoral compartments. To date, there is very little information regarding the knee mechanics during gait after bi-compartmental knee replacement. The purpose of the study was to evaluate knee strength and mechanics during level walking after knee replacement. Methods: Ten healthy control subjects and eight patients with unilateral bi-compartmental knee replacement participated in the study. Maximal isokinetic concentric knee extension strength was evaluated. 3D kinematic and kinetic analyses were conducted for level walking. Paired Student t-test was used to determine difference between surgical and non-involved limbs. One way MANOVA was used to determine difference between surgical and control groups. Findings: The surgical knee exhibited less peak torque and initial abduction moment than both the noninvolved and control limbs (P < 0.05). The non-involved limb had less knee extension at stance and greater knee extensor moment during push-off than both the surgical and control limbs (P < 0.05). No differences were found for other typical knee mechanics among the surgical, non-involved, and control limbs during walking (P > 0.05). Interpretations: Patients with bi-compartmental knee replacement exhibited good frontal plane knee mechanics and were able to produce the same level of knee extensor moment as healthy control limbs during walking. While showing some compensatory patterns during walking, patients with bi-compartmental knee replacement largely exhibited normal gait patterns and knee mechanics. Published by Elsevier Ltd. 1. Introduction Osteoarthritis (OA) is a cartilage degenerative disease and causes more disability with respect to mobility than any other single disease in the elderly (Guccione et al., 1994). Knee OA typically affects joints in a non-uniform manner; the medial compartment of the knee is most frequently affected in both men and women (Windsor and Insall, 1994). Furthermore, the three most common areas of knee OA distribution are medial compartment, patellofemoral compartment, and medial/patellofemoral compartment overlap (McAlindon et al., 1992). The predominance of medial knee OA is likely due to the high medial forces generated during weightbearing activities (e.g. walking) (Morrison 1970; Schipplein and Andriacchi, 1991). It is believed that knee adduction torque is strongly associated with risk of medial knee OA progression (Miyazaki et al., 2002). * Corresponding author. address: hwang2@bsu.edu (H. Wang). Knee osteoarthritis (OA) and problems associated with varus knee alignment are the primary factors leading to total knee replacements (TKR). However, the damage associated with these conditions is often limited to cartilage degeneration in the medial and patellofemoral compartments. One alternative to the TKR, especially for more active patients, is uni-compartmental knee (UKR) replacement which leaves the lateral knee compartment and cruciate ligaments intact while replacing only the affected medial compartment. UKR leads to faster recovery times, less bone loss, and better knee kinematics than TKR (Swienckowski and Pennington, 2004; Banks et al., 2005; Fuchs et al., 2005). However, UKR does not treat the OA at patellofemoral joint which is common in those patients with compromised medial compartments. As it was found that patellofemoral joint arthritis is often associated with medial compartment arthritis (McAlindon et al., 1992), a bi-compartmental knee replacement (BKR) was introduced to treat the medial compartment and patellofemoral joint arthritis (Journey Deuce system TM, Smith & Nephew Inc., Memphis, TN, USA). Similar to a UKR, the BKR preserves the lateral compartment of the knee and cruciate ligaments while also addressing the /$ - see front matter Published by Elsevier Ltd. doi: /j.clinbiomech

3 752 H. Wang et al. / Clinical Biomechanics 24 (2009) patellofemoral articulation. With intact lateral compartment and cruciate ligaments, the BKR may lead to faster recovery and better functional outcomes. However, objective data are lacking to support this hypothesis. The primary purpose of this study was to evaluate knee mechanics during level walking in patients who have undergone unilateral BKR system. The secondary purpose was to evaluate knee strength in patients who have undergone unilateral BKR system. As the BKR surgery results in less loss of bone and retains the lateral compartment and cruciate ligaments of the knee, a return to normal knee extensor strength and normal knee kinematics was expected after surgery. In addition, the knee extensor moment arm was expected to be similar to control knees. Therefore, it was hypothesized that there would be no differences of net knee extensor moments between the BKR, non-involved, and healthy control limbs during knee strength tests and level walking. Also, the BKR surgery is expected to result in good longitudinal alignment between the femur and tibia. The mechanical axis is expected to pass through the center of the knee joint. Therefore, there would be no differences of knee abduction/adduction knee joint moments between the BKR, non-involved, and healthy control limbs during level walking. The combination of these potential improvements may also lead to a reduction or absence of compensatory gait patterns. Specifically, it was expected there would be no differences of step length, single support time, and knee flexion angles between the BKR, non-involved, and healthy control limbs during level walking. 2. Methods Ten healthy subjects formed a control group. Eight participants from a pool of patients who had been diagnosed with OA in the medial and patellofemeral compartments and undergone unilateral BKR surgery (performed by the same surgeon) with the Journey Deuce TM system (Smith & Nephew Inc., Memphis, TN, USA) formed the BKR group. In addition, these BKR patients were selected based on the following criteria: (a) no history of major diagnosed health problems or injuries that would affect performance; (b) medical clearance from the surgeon; (c) no current symptoms of pain, injury, or muscle soreness; (d) have both the functional score and knee society score greater than 90; (e) no orthopedic problems, OA symptoms, or pain in the non-involved knees; (f) no OA symptoms in the lateral compartment of the involved limb. All patients had gone through a standard rehabilitation program after the surgery. The means and SDs of age, body mass, body height of the BKR and control groups and post-operative time, knee society clinical score and functional score of the BKR limbs are reported in Table 1. Institutional Research Board (IRB) approval was obtained prior to commencing the study. The subjects came to the Ball State University Biomechanics Laboratory on two separate occasions with one week apart. During Table 1 Means and SDs of age, body mass, body height, and walking speed of the BKR and control groups and post-operative time, knee society clinical score and functional score of the BKR limbs. BKR (n = 8) Control (n = 10) Age (yrs) 66 (4) 58 (8) Body mass (kg) 100 (13) 98 (8) Height (cm) 176 (5) 180 (9) Walking speed (m/s) 1.28 (0.07) 1.35 (0.09) Post-operative time (mo.) 14 (4) NA Knee society clinical score 98 (4) NA Functional score 100 (0) NA the first session the subject s maximal knee extensor torque was measured using a CYBEX NORM TM isokinetic dynamometer (CYBEX International, Inc., NY, USA) at 60 deg/s. During the second session, 3-dimensional (3D) kinematic and kinetic analyses were conducted while the subjects performed five trials of level walking at self-selected pace. Ten VICON TM F40 cameras (Vicon, Lake Forest, CA, USA) at 100 Hz were used to capture the spatial locations of reflective markers on the participant. The standard full body plug-in gait model was used for the placement of the markers on the body. A VICON Workstation TM system (V 4.2) was used to generate 3D coordinates of the reflective markers. Two AMTI TM force platforms (Model BP , Advanced Medical Technology, Inc., Watertown, MA, USA) were used to collect the ground reaction forces at 1000 Hz. The following dependent variables were analyzed: maximum isokinetic concentric knee extension (KE) torque, knee flexion at foot strike, peak knee flexion, extension and adduction angles at stance, peak knee abduction moment at foot strike, peak knee adduction moment during stance, peak knee extensor moment at push-off, walking speed, single support time, and step length. In addition, knee extension torques during strength testing and knee moments during level walking were normalized to body mass. One-way multiple analysis of variance (MANOVA) was used to determine kinematic and kinetic difference between BKR and control groups. Paired Student t-test was used to determine kinematic and kinetic difference between non-involved limbs and BKR limbs. Post-hoc power analysis was performed to determine the effect sizes and achieved power of each dependent variable. Significant level was set at Results Effect sizes and achieved power of dependent variables were reported in Table 2. During strength testing, BKR limbs exhibited less KE torque than both the non-involved and control limbs (P < 0.05) (Fig. 1). During level walking, BKR limbs exhibited smaller peak of initial adduction moment at foot strike than both the non-involved and control limbs (P < 0.05) (Table 3). Non-involved limbs had less knee extension at mid-stance and greater normalized knee extensor moment at push-off (Table 3) than the BKR and control limbs (P < 0.05). No difference was found for walking speed between the two groups (P > 0.05) (Table 1). No differences were found for knee flexion at foot strike, peak knee flexion, adduction, and adduction moment during stance between the control, BKR and non-bkr limbs (P > 0.05) (Table 2). No differences were found for Table 2 Achieved power and effect size of dependent variables. Dependent variables Effect size (based on means) Achieved power Knee flexion at foot strike Peak knee flexion during 1st half of stance Peak knee extension Peak knee adduction Peak knee extensor moment at pushoff Peak initial knee abduction moment at Foot strike Peak adduction moment Step length Single support time Max isokinetic concentric knee extensor torque

4 H. Wang et al. / Clinical Biomechanics 24 (2009) Fig. 1. Maximum isokinetic concentric knee extension torque for control, noninvolved, and the BKR limbs. step length, single support time between the BKR and non-involved limbs (P > 0.05) (Table 3). 4. Discussion In summary, the purpose of this study was to evaluate knee mechanics during level walking and knee strength in patients who had undergone unilateral BKR system. As the BKR surgery results in less loss of bone and retains the lateral compartment and cruciate ligaments of the knee, it was expected that knee mechanics and knee strength of the unilateral BKR subjects would not be different than those of their non-involved limbs or those of healthy control subjects. The BKR limbs exhibited less isokinetic knee extensor strength than both of the non-involved limbs and limbs from the control group. This result suggests that at approximately one year postsurgery, BKR knee extensor strength has not yet returned to normal. As none of the patients involved in the study had participated in a strength training program, it may be possible to elicit additional improvements in knee extensor strength for the BKR limbs if the patients participated in a regular strength training program after the surgery. It was also hypothesized that there would be no difference of knee extensor moment at push-off among the BKR, non-involved, and control limbs. This hypothesis was partially supported as there was no difference between the BKR and control limbs. This finding indicated that the BKR limb was able to produce a similar knee extensor moment during normal walking compared to that of healthy control limbs. Although the maximum knee extensor strength of the BKR limb was still less than both the non-involved and control limbs, it appears that during daily activities (e.g. normal walking), the BKR limb had no limitation in its ability to perform in a similar fashion to the normal control limb. On the other hand, the non-involved limb exhibited less knee extension and greater knee extensor moment at push-off than the BKR and control limbs. This finding is thought to be a result of a possible retention of the gait pattern formed prior to the surgery. The development of knee OA and the associated knee pain may have led to less usage of the involved limb and increased the dependency on the non-involved limb during gait. This would potentially lead to a reliance on the non-involved limb to produce a greater knee extensor moment at push-off to compensate for the painful knee. While this explanation seems plausible, this study does not include any pre-surgery analyses, therefore, it is not possible to determine if this walking pattern was present before the BKR. One of the goals of BKR surgery is to correct the frontal plane knee deformity. Patients with medial and patello-femoral knee osteoarthritis typically have significant varus deformity. It was reported that BKR surgery effectively corrects varus deformity and restores good knee alignment (Rolston and Siewert, 2008). In the current study, despite the lack of a direct measure on frontal plane knee alignment, the finding of no differences of peak knee adduction angle and moment between the BKR, non-involved, and control limbs, and the results in the previous work on BKR efficacy in alignment correction (Rolston and Siewert, 2008), suggested that good frontal plane knee mechanics during walking corresponded to a good knee alignment. As it was reported that both the increased peak knee adduction moment (Baliunas et al., 2002; Hunt et al., 2006; Milner and O Bryan, 2008) and increased peak adduction angle (Astephen and Deluzio, 2005) were associated with medial knee OA, the absence of an increased peak knee adduction angle and moment in the BKR limb indicates that frontal plane knee mechanics during walking has returned to normal after the BKR surgery. It is also reported that higher abduction knee moment exhibited at heel contact is often associated with medial knee OA and possibly used as a strategy to reduce the high knee adduction moment occurring at late stance for reducing pain (Mundermann et al., 2005). In this study, the reduction of peak abduction knee moment at heel contact likely indicates a positive change of gait pattern after BKR surgery as patients with BKR did not exhibit the typical pain avoidance compensatory pattern typically seen in individuals with knee OA. Table 3 Means and SDs of typical variables of knee mechanics and gait kinematics at stance (flexion at foot strike, peak flexion, extension, and adduction, peak extensor moment, initial peak of adduction moment at foot strike, and adduction moment, step length, and single support time) of the control, non-involved, and BKR limbs. Variables of knee mechanics and gait kinematics Control Non-involved BKR Knee mechanics Knee flexion at foot strike ( ) 6(3) 7(3) 7(3) Peak knee flexion during 1st half of stance ( ) 21(5) 20(5) 17(7) Peak knee extension ( ) 1(5) *** 5(4) *,*** 3(3) * Peak knee adduction ( ) 7(3) 6(4) 7(4) Peak knee extensor moment at push-off (N m/kg) 0.25(0.09) *** 0.37(0.15) *,*** 0.27(0.11) * Initial peak of knee adduction moment at foot strike (N m/kg) 0.06(0.06) ** 0.06(0.08) * 0.01(0.06) *,** Peak adduction moment (N m/kg) 0.44(0.14) 0.42(0.13) 0.53(0.16) Gait kinematics Step length (m) 0.73 (0.06) 0.69(0.05) 0.67(0.06) Single support time (s) 0.42 (0.02) 0.41(0.02) 0.40(0.01) * Indicates significant difference (P < 0.05) between the BKR and non-involved limbs. ** Indicates significant difference (P < 0.05) between the BKR and the control limbs. *** Indicates significant difference (P < 0.05) between the non-involved and control limbs.

5 754 H. Wang et al. / Clinical Biomechanics 24 (2009) Finally, level walking was well performed by the BKR group. This was supported by the similar walking speed between the BKR and control groups and similar knee joint kinematics and gait kinematics between the BKR and non-involved limbs. In conclusion, the BKR patients exhibited good frontal plane knee mechanics and were able to produce the same level of knee extensor moment as healthy control limbs during walking. While showing some compensatory patterns, the BKR patients had largely returned to normal gait patterns and knee mechanics. Some limitations must be addressed here. Firstly, the BKR patients preoperative gait data were lacking. It is not known how much of an improvement achieved in leg strength and gait after the BKR surgery. Secondly, we did not include a TKR group in the study. Thus, we could not make comparisons between the BKR and TKR limbs during gait. Future studies should focus on the above two issues. Conflict of interest There is no conflict of interest in this study. Acknowledgement BSU S.E.E.T Fund. References Astephen, J.L., Deluzio, K.J., Changes in frontal plane dynamics and the loading response phase of the gait cycle are characteristic of severe knee osteoarthritis application of a multidimensional analysis technique. Clin. Biomech. 20 (2), Baliunas, A.J., Hurwitz, D.E., et al., Increased knee joint loads during walking are present in subjects with knee osteoarthritis. Osteoarthr. Cartilage 10 (7), Banks, S.A., Fregly, B.J., et al., Comparing in vivo kinematics of unicondylar and bi-unicondylar knee replacements. Knee Surg Sports Traumatol. Arthrosc. 13 (7), Fuchs, S., Tibesku, C.O., et al., Clinical and functional comparison of uni- and bicondylar sledge prostheses. Knee Surg Sports Traumatol. Arthrosc. 13 (3), Guccione, A.A., Felson, D.T., et al., The effects of specific medical conditions on the functional limitations of elders in the Framingham Study. Am. J. Public Health 84 (3), Hunt, M.A., Birmingham, T.B., et al., Associations among knee adduction moment, frontal plane ground reaction force, and lever arm during walking in patients with knee osteoarthritis. J. Biomech. 39 (12), McAlindon, T.E., Cooper, C., et al., Knee pain and disability in the community. Br. J. Rheumatol. 31 (3), Milner, C.E., O Bryan, M.E., Bilateral frontal plane mechanics after unilateral total knee arthroplasty. Arch. Phys. Med. Rehabil. 89 (10), Miyazaki, T., Wada, M., et al., Dynamic load at baseline can predict radiographic disease progression in medial compartment knee osteoarthritis. Ann. Rheum. Dis. 61 (7), Morrison, J.B., The mechanics of the knee joint in relation to normal walking. J. Biomech. 3 (1), Mundermann, A., Dyrby, C.O., et al., Secondary gait changes in patients with medial compartment knee osteoarthritis: increased load at the ankle, knee, and hip during walking. Arthritis Rheum. 52 (9), Rolston, L., Siewert, K., Assessment of knee alignment after bicompartmental knee arthroplasty. J. Arthroplasty (Epub ahead of print). Schipplein, O., Andriacchi, T., Interaction between active and passive knee stabilizers during level walking. J. Orthop. Res. 9 (1), Swienckowski, J.J., Pennington, D.W., Unicompartmental knee arthroplasty in patients sixty years of age or younger. J. Bone Joint Surg. Am. 86-A (Suppl. 1(Pt 2)), Windsor, R., Insall, J., Surgery of the knee. In: Sledge, C., Ruddy, S., Harris, E., Kelley, W. (Eds.), Arthritis Surgery. W.B. Saunders, Philadelphia, pp