Original Article
Differences between Standard and Minimally Invasive Parapatellar Surgical Approaches for Total Knee Arthroplasty in the Tasks of Sitting and Standing Sarah L. Jarvis, MS1 Amanda K. Johnson-Wo, BS1 Brian R. Onstot, BS2 Manoshi Bhowmik-Stoker, PhD3 M. Wade Shrader, MD4 Marc C. Jacofsky, PhD1 David J. Jacofsky, MD5 1 Department of Research, The CORE Institute, Sun City West, Arizona 2 Arizona College of Osteopathic Medicine, Midwestern University,
Glendale, Arizona 3 Department of Orthopaedics, Stryker, Mahwah, New Jersey 4 Department of Orthopaedics, Phoenix Children’s Hospital, Phoenix,
Address for correspondence and reprint requests David J. Jacofsky, MD, Department of Research, The CORE Institute, 3010 West Agua Fria Freeway, Phoenix, AZ 85027 (e-mail:
[email protected]).
Arizona
J Knee Surg
Abstract
Keywords
► total knee arthroplasty ► minimally invasive ► mini-parapatellar ► standard parapatellar ► quadriceps weakness
Minimally invasive total knee arthroplasty (TKA) aims to enhance functional recovery and minimize trauma to the knee extensor mechanism through quadriceps sparing techniques. Few have studied the effect of TKA surgical approach on activities of daily living. Stand-to-sit-down (STSD) and sit-to-stand-up (STSU) activities are challenging for patients in constrained scenarios where upper body support is limited. In this study, 60 subjects diagnosed with primary osteoarthritis undergoing TKA were randomized to receive the standard parapatellar (SP) or mini-parapatellar (MP) surgical approach performed using computer navigation. All received identical postoperative orders, hospitalization, and physical therapy. Before surgery and at 2, 4, and 6 months postoperatively, the STSD and STSU kinetics and kinematics showed differences for both groups in comparison with controls, but the two were essentially indistinguishable throughout all time points.
Osteoarthritis (OA) is the most common form of arthritis1 and continues to be a leading cause of disability in the United States.2 The knee is one of the most frequently affected joints,3 which can significantly impact activities of daily living (ADLs). Total knee arthroplasty (TKA) has become a common and widely used treatment to minimize pain and restore function within individuals with advanced knee arthritis. Several surgical approaches are available for surgeons to select including the standard parapatellar (SP) and miniparapatellar (MP). Much controversy exists in the world of knee arthroplasty as to which procedure provides the best clinical and functional outcomes for patients. Historically, the
SP approach described by Insall4 has been the most commonly used because of its ease of access to the joint space4,5 and generally good clinical and radiographic outcome over the past few decades.6–9 However, this approach, involving a medial parapatellar arthrotomy (an 8-cm split in the central portion of the quadriceps tendon) and eversion of the patella, has been associated with increased postoperative pain, longer hospital stays, and decreased quadriceps strength,5,10–14 leading some to question whether alternative surgical approaches may be more beneficial to recovery. Minimally invasive procedures have been developed to minimize perioperative complications and decrease the
received August 27, 2012 accepted September 12, 2012
Copyright © by Thieme Medical Publishers, Inc., 333 Seventh Avenue, New York, NY 10001, USA. Tel: +1(212) 584-4662.
DOI http://dx.doi.org/ 10.1055/s-0032-1329718. ISSN 1538-8506.
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5 Department of Research, The CORE Institute, Phoenix, Arizona
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approach, which imposes less disruption to the extensor mechanism, would allow patients to sit and stand more effectively than patients with an SP approach. We hypothesized that the MP approach would allow subjects to sit and stand in a manner comparable with a matched control group earlier in the recovery period than the SP approach.
Patients and Methods Subjects The approval for this study was provided by the Sun Health Institutional Review Board. Subjects who volunteered for the study were recruited at The CORE Institute by the surgeons performing the TKA procedures and were provided informed consent before data collection. The inclusion criteria in the study required participants to have a primary diagnosis of knee OA and to be scheduled to receive TKA. Subjects presenting with other significant pathology in the lower extremities or any other diagnosed gait disorders were excluded from the study. Participants in the study were prospectively randomized to receive the SP or MP surgical approach. All TKA procedures were performed by one of three fellowship-trained orthopedic surgeons experienced in both approaches, with an average of 180 TKAs performed each year, familiar with each other’s techniques, and using specifically agreed on approaches. Prostheses (Stryker Triathlon posterior-stabilized TKA) were sized appropriately for each subject and were implanted using computer navigation to minimize differences in implant position as a result of surgical approach. All patients received standard postoperative hospital orders, clinical pathways, and therapy.
Surgical Technique For the SP approach, a 12.5-cm skin incision was made longitudinally over the knee from 2 cm proximal to the superior pole of the patella on the proximal end to the subchondral tibial plateau on the distal end. An SP arthrotomy was used to expose the joint distally and the quadriceps tendon was split longitudinally 40% of the distance in the lateral direction from the medial side of the tendon. The patella was everted to gain additional joint exposure. For the MP approach, a 12.5-cm skin incision was made longitudinally over the knee from 2 cm proximal to the superior pole of the patella on the proximal end to the subchondral tibial plateau on the distal end. A parapatellar arthrotomy was used to expose the joint space. The quadriceps tendon was split longitudinally 10% of the distance in the lateral direction from the medial side for a distance of only 3 cm. The patella was lateralized, but not everted, to gain additional joint exposure.
Biomechanical Analysis Motion analysis was conducted on each subject preoperatively, and at 2, 4, and 6 months, postoperatively. A 10-camera passive marker system (Motion Analysis Corp., Santa Rosa, CA) collected three-dimensional kinematic data from 28 retro-reflective spherical markers affixed to each subject’s
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recovery period after TKA, through improved early quadriceps function.5,13–20 This is perhaps the most important functional advantage of minimally invasive TKA, although it seems that this advantage may be short lived with appropriate rehabilitation. Clinical differences are often indistinguishable between SP and minimally invasive groups 6 months after surgery,5,14,16,18–24 and such procedures are not without their own set of complications. Gandhi et al25 showed that minimally invasive TKA has been associated with a significantly increased rate of local surgical complications. Other reports have also indicated issues with malalignment or bone injuries due to the limited access to the joint space.5,13,14,16,18,22 The achievement of proper alignment and cementation of components can be more difficult with minimally invasive TKA due to the smaller incision.26,27 However, this does not seem to be as much of an issue when minimally invasive TKA is paired with computer navigation, which helps to eliminate some of the difficulty encountered with visualization and the use of jigs with a limited exposure.10,26 Although minimally invasive TKA is believed to result in decreased postoperative pain, blood loss, length of hospital stay, and tourniquet time, conflicting reports in the literature on these differences5,14,16,17,19,24 may indicate that surgeon experience and postsurgical care are more influential than the TKA approach used. Strict inclusion criteria for minimally invasive procedures may also impose a bias toward patients who are more likely to have positive outcomes after TKA regardless of the approach used.27 Minimally invasive procedures are also gaining popularity among patients because of a more favorable cosmetic result14,15 and the misconceived notion of having a less-involved surgery.13,14,27 However, a minimally invasive procedure is not defined by the length of skin incision, but rather by the extent of disruption to the joint structures.14,17 Many are quick to compare minimally invasive TKA to arthroscopy on the basis of the reduced incision size, but minimally invasive TKA further inhibits visualization of the joint space increasing the risk of complications during surgery.27 Despite the complications that may arise in doing a complex surgery through a smaller incision, minimally invasive TKA has continued to evolve over the past decade with generally positive outcomes.5,13–20 In comparison with other minimally invasive approaches, the MP approach tends to be the easiest for surgeons to adopt due to its similarity to the standard approach,15 and may, therefore, become the most widely used technique should minimally invasive TKA become a new standard. Few studies have evaluated the effect of minimally invasive TKA on ADLs, focusing instead on clinical scores, range of motion (ROM), and radiographic alignment.5,13,16,18–20,23,24 Sitting and standing are frequent ADLs that require greater joint forces and ROM than level walking.28 The ability to complete these movements, independent of upper body support, is hindered immediately after TKA,28–30 which is likely due to quadriceps weakness that is frequently seen postoperatively.11,31 Therefore, it was the aim of this study to determine whether a minimally invasive parapatellar TKA
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Results Subject Characteristics
Fig. 1 Control subject in the starting position for the sit-to-stand-up movement on an armless stool with one foot comfortably placed on each force platform.
body following a modified Helen Hayes marker set.32 Before analysis, each subject performed static calibration trials of each limb to facilitate calculation of the hip joint centers of rotation for kinematic analysis. To complete the stand-to-sitdown (STSD) and sit-to-stand-up (STSU) tasks, subjects were instructed to sit on an armless, backless stool adjusted to yield approximately 90 degrees of knee and hip flexion such that the knee and ankle were in line, with each foot comfortably placed on separate force platforms (AMTI OR6, Advanced Mechanical Technology Inc., Watertown, MA) (►Fig. 1). Subjects were then asked to rise to a standing position in one continuous motion, without significant preparatory arm or trunk motion. After sufficient time was allowed for the subject to become stabilized, they were asked to descend to a seated position on the stool. Subjects were instructed to perform these tasks without upper extremity assistance or rotation of the trunk. Trials were excluded from analysis if the specific task instructions were not followed. A minimum of five cycles of both STSD and STSU motions were recorded. Data collection, smoothing, and verification were done using EVaRT 5.0.1 software (Motion Analysis Corp., Santa Rosa, CA) with a 6-Hz Butterworth filter. Height and weight normalization and ensemble averaging of trial and group data were calculated using OrthoTrak 6.6.1 (Motion Analysis Corp., Santa Rosa, CA) with the “Original Knee Axis,” “Functional Hip Center,” and “Linked Foot Axes” options selected. Custom event detection scripts were developed and run in MatLab
All participants enrolled in the study met the specified inclusion criteria; however, several subjects were excluded at each time point due to missed appointments or invalid data. Although many subjects were highly functional, noticeable difficulty in completing a task was not used as a basis for exclusion in either group. The numbers of subjects able or unable to perform the STSD or STSU tasks as instructed were not significantly different between test groups at any time point (p 0.333). However, there were significantly more missed appointments at the 4-month time point in the MP group than the SP group (p ¼ 0.013). No significant differences in demographics (►Table 1) were found in either test group when compared with the control population (p 0.100). Patients who received a subsequent contralateral TKA less than 2 months apart were excluded in both groups, bringing group totals to N ¼ 28 knees for the SP group and N ¼ 32 knees for the MP group.
Stand-to-Sit-Down Movement Preoperative Analysis Of the 30 patients enrolled in the MP group and 28 patients enrolled in the SP group, 18 MP subjects and 19 SP subjects successfully completed the sitting task at the preoperative Table 1 Subject demographics Control
MP
SP
Females
12
17
19
Males
10
15
9
Age, mean SD
68.3 5.4
69.8 6.8
69.7 7.6
BMI, mean SD
28.8 3.7
31.7 5.5
29.2 5.2
Abbreviations: BMI, body mass index; MP, mini-parapatellar; SP, standard parapatellar. The Journal of Knee Surgery
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R2008a (The Mathworks Inc., Natick, MA). Data for STSD and STSU movements were analyzed in two simplified phases from that of described by Kerr et al33 that applied to each movement cycle. A preparatory phase and a sitting/rising phase, where the transition between these phases was defined as the point where knee angular velocity exceeded a threshold of 12 degrees/s, based on the suggestions by Kerr et al and optimizations through prestudy data collection and analysis. The start and end of each trial were defined using trunk tilt velocity thresholds of 5 and – 5 degrees/s, respectively, were also based on Kerr et al.33 A one-way analysis of variance was performed (SPSS 14.0, SPSS Inc., Chicago, IL), followed by post hoc comparisons using Tukey and Dunnett tests, where appropriate to identify significant differences between test group means for α ¼ 0.05 based on the homogeneity of variance test. χ2 tests were also used to compare the number of subjects in each group that were unable to complete the STSD or STSU tasks as instructed.
Differences between SP and MP Surgical Approaches for TKA
2-Month Analysis Nineteen MP subjects and 17 SP subjects were included in the analysis of sitting movement at the 2-month time point. The SP group continued to have an increased knee flexion angle at the beginning of the STSD movement (p ¼ 0.044), whereas the MP group continued to have more knee internal rotation than both the SP (p ¼ 0.038) and control groups (p ¼ 0.001). Hip flexion angle, which was not significant preoperatively, was now increased only significantly for the SP group in comparison with controls (p ¼ 0.027) during the sitting phase. Both SP and MP groups maintained a lower knee extension moment at the beginning of the preparatory phase (p < 0.001). Hip abduction and external rotation moments remained significantly decreased compared with controls throughout the preparatory phase for both SP (p 0.001) and MP (p < 0.008) groups. Both groups had a period of net hip adduction moment near the end of the preparatory phase, which was present preoperatively but now had a period of net hip internal rotation moment as well throughout the preparatory phase as compared with the net abduction and external rotation moments seen in the control group. Both groups now exhibited decreased peak knee power during the preparatory phase (p 0.003) in comparison with controls. Maximum vertical GRF remained lower for the SP group (p ¼ 0.027) as compared with the control group during the preparatory phase.
4-Month Analysis Fourteen MP subjects and 18 SP subjects were included in the analysis of sitting movement at the 4-month time point. Hip rotation ROM was now significantly greater for only the MP group when compared with controls (p ¼ 0.012) throughout the sitting movement cycle. Knee extension moment remained lower than control-like values during the preparatory phase for both the SP (p ¼ 0.024) and MP (p ¼ 0.041) groups. Peak knee power remained decreased during the preparatory phase for only the SP group in comparison to controls (p ¼ 0.039). Hip power was now significantly greater for the MP group compared with the SP group (p ¼ 0.049) during the sitting phase. GRFs returned to control-like values for both the SP and the MP group. The Journal of Knee Surgery
6-Month Analysis Sixteen MP subjects and 19 SP subjects were included in the analysis of sitting movement at the 6-month time point. There was now less external rotation at the hip in the SP group compared with controls during the sitting movement (p ¼ 0.018). Peak knee extension moment remained smaller for the SP group compared with controls during the preparatory phase (p ¼ 0.035). Similar to the 2-month time point, hip abduction moment was decreased from control-like values but now only significantly for the SP group (p ¼ 0.007) during the preparatory phase. Both SP and MP groups continued to exhibit a period of net hip adduction moment near the end of the preparatory phase as opposed to controls who exhibited only net abduction moment during the preparatory phase, which was also seen at 2 months after surgery. Hip internal rotation moment was significantly lower throughout the STSD movement cycle, but now only for the SP group when compared with controls (p ¼ 0.029). Although no differences were seen in hip power at 2 and 4 months after surgery, the SP group had a significant decrease in hip power just before the preparatory-sitting phase transition when compared with controls (p ¼ 0.035) at 6 months. There were no significant differences in hip or knee output parameters between the SP and MP groups at 6 months postoperation in the STSD movement.
Sit-to-Stand-Up Movement Preoperative Analysis Of the 30 patients enrolled in the MP group and 28 patients enrolled in the SP group, 19 MP subjects and 15 SP subjects successfully completed the rising task at the preoperative time point as instructed. Before surgery, knee flexion was increased significantly for both groups compared with controls at the end of the rising phase (p 0.025). Both groups had significantly less knee abduction compared with controls throughout both the preparatory and rising phases (p 0.049). Knee internal rotation during the preparatory and early rising phases was significantly higher for the MP group compared with both the control (p ¼ 0.003) and SP (p ¼ 0.032) groups and was the only significant parameter in comparing the MP group with the SP group, preoperatively. In comparison with controls, both SP and MP groups had decreased hip abduction angles during the preparatory phase (p 0.041). Although only the SP group experienced significant alterations compared with controls, both groups had decreased hip external rotation during the preparatory phase (p ¼ 0.027).Throughout the rising phase, the knee had decreased internal rotation moments in both groups compared with controls (p 0.005). Peak knee extension moment (►Fig. 2) was significantly lower during the rising phase for the MP and SP groups compared with controls (p 0.031). Hip rotation kinetics tended to be dominated by an internal rotation moment for both groups as opposed to the control group, which had more external rotation moment throughout the rising phase. Hip abduction moment was significantly decreased for both groups throughout the rising phase compared with controls (p 0.003) with a period of net
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time point as instructed. Joint angles with significant differences before surgery were knee flexion and knee internal rotation, where the SP group had increased knee flexion compared with controls at the beginning of the STSD movement (p ¼ 0.034) and the MP group had less external rotation at the knee compared with both the SP (p ¼ 0.047) and control groups (p ¼ 0.004). The MP group had a lower knee extension moment than controls during the preparatory phase (p ¼ 0.042). In comparison with controls, both MP and SP groups had decreased hip external rotation (p 0.026) and abduction (p 0.039) moments during the preparatory phase. Compared with controls, the SP group had decreased peak hip power during the preparatory phase (p ¼ 0.034). Maximum vertical ground reaction forces (GRF) during the preparatory phase was lower for only the SP group compared with controls (p ¼ 0.025).
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Fig. 2 Knee extension moment (N/kg) during sit-to-stand up preoperatively and at 2, 4, and 6 months postoperatively as a function of percent gait cycle. Vertical lines indicate the distinction between the preparatory and rising phases for each group as defined by knee angular velocity greater than 12 degrees/s. MP, mini-parapatellar; SP, standard parapatellar.
adduction moment at the beginning of the rising phase as opposed to nearly all adduction moment demonstrated by controls. Peak knee power (►Fig. 3) and peak vertical GRF were both significantly lower during the rising phase for the MP and SP groups compared with controls (p 0.016).
2-Month Analysis Nineteen MP subjects and 15 SP subjects were included in the analysis of rising movement at the 2-month time point. The transition between the preparatory and the rising phases occurred earlier in the overall rising movement in the MP group than in the control group (p ¼ 0.009). Knee flexion at the end of the rising phase was now significantly higher for only the SP group compared with controls (p ¼ 0.006). During both the preparatory and early rising phases, the knee remained less externally rotated in the MP group than in the control group (p 0.030). Both SP and MP groups continued to have significantly decreased knee extension moments (p < 0.001) while rising compared with controls, but the MP group also had a significantly higher knee extension moment compared with the SP group (p ¼ 0.025) just before the preparatory-rising phase transition (►Fig. 2). Knee abduction moment was now lower than control-like values at the end of the preparatory phase in both groups, but was only significant for the SP group (p ¼ 0.007). At the beginning of the rising phase, both SP and MP groups continued to have decreased internal rotation moments at the knee (p 0.001), whereas the MP group also exhibited a period of net external rotation moment during early rising. There was now a significantly greater amount of hip flexion in the SP group when compared with controls (p ¼ 0.001) during the rising
phase of the STSU movement. Hip abduction and external rotation moments remained significantly decreased for both SP (p 0.009) and MP (p 0.001) groups in comparison with controls during the rising phase, where there continued to be a period of net adduction moment and net internal rotation moment at the start of the rising phase as compared with net abduction and greater external rotation as seen in the control group. Knee power remained significantly decreased for both SP and MP groups compared with controls (p < 0.001) (►Fig. 3). Vertical GRF remained smaller at the beginning of the rising phase for both SP and MP groups (p < 0.001), but propulsive GRF now indicated less posterior-directed force during the rising phase for both groups in comparison with controls (p ¼ 0.043).
4-Month Analysis Fourteen MP subjects and 21 SP subjects were included in the analysis of rising movement at the 4-month time point. The MP group continued to reach the transition between the preparatory and rising phases sooner than controls (p ¼ 0.015). No differences in joint angles were seen between groups or in comparison with controls at 4 months after surgery. As seen at previous time points, there continued to be a significantly lower peak knee extension moment (p 0.002), as well as significantly lower peak knee power (p < 0.001), while rising for both SP and MP groups compared with controls (►Figs. 2, 3). Similar to the 2-month time point for STSU, hip abduction moment continued to be significantly lower for both SP (p 0.026) and MP (p 0.033) groups during the rising phase, where there was a period of net adduction moment at the start of the rising phase as The Journal of Knee Surgery
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Fig. 3 Knee power (W/kg) during sit-to-stand up preoperatively and at 2, 4, and 6 months postoperative as a function of percent of gait cycle. Vertical lines indicate the distinction between the preparatory and rising phases for each group as defined by knee angular velocity greater than 12 degrees/s. MP, mini-parapatellar; SP, standard parapatellar.
compared with net abduction moment in the control group. Only the MP group continued to have a significantly greater hip external rotation moment at the beginning of the rising phase compared with the controls (p ¼ 0.030). No significant differences were seen in GRFs compared with controls or between the SP and MP groups at 4 months postoperation in the STSU movement.
6-Month Analysis Sixteen MP subjects and 20 SP subjects were included in the analysis of rising movement at the 6-month time point. The knee was less externally rotated than the controls during the preparatory phase for both groups, but was only significant for the MP group (p ¼ 0.041), similar to 2 months after surgery. The peak knee extension moment while rising (►Fig. 2) remained significantly lower for both SP and MP groups compared with controls (p 0.003), as did the knee abduction moment at the end of the preparatory phase when comparing the SP group with controls (p ¼ 0.018). Hip abduction moment continued to be significantly decreased from that of controls for both SP (p 0.005) and MP (p ¼ 0.011) groups while rising from a seated position. Similar to the 2-month time point, hip external rotation moment remained lower with a smaller peak moment while rising in both SP (p < 0.001) and MP (p ¼ 0.013) groups in comparison with controls. Both SP and MP groups continued to exhibit smaller peak knee power than the controls during the rising phase (p 0.005) (►Fig. 3). Similar to the 2-month time point, the vertical GRF was significantly smaller at the beginning of the rising phase for the SP group as compared with controls (p < 0.004); however, it was not significantly The Journal of Knee Surgery
different for the MP group. No significant differences between the SP and MP groups were seen in any of the hip or knee output parameters at 6 months postoperation.
Discussion It was hypothesized that the MP approach would allow subjects to sit and stand in a manner more similar to a matched control group earlier in the recovery period than subjects who had the SP approach. The results of this study indicate that very few significant differences exist at the knee and hip in kinematic and kinetic parameters between SP and MP surgical approaches as early as 2 months post-TKA, but differences from controls are still evident at 4 and 6 months in both sitting and standing tasks. Surprisingly, differences in knee ROM, moments, and powers are minimal, but compensations at neighboring joints seem to suggest different compensatory mechanisms for the reduced knee function in each group. Elderly individuals tend to maximize their available quadriceps strength when rising from a chair,34 making quadriceps weakness following TKA an important issue. The ability to rise from a chair has also been said to be one of the most demanding ADLs,35 involving high joint forces and extension moments, accurate coordination, and control of balance.28,36–38 Faster recovery of quadriceps weakness has been seen clinically in the early period following minimally invasive TKA as opposed to the SP approach.5,15,17–20 Although previous studies have analyzed the sitting and standing movements in healthy individuals,39,40 we are unaware of any previous study analyzing this movement specifically comparing minimally invasive TKA to the standard approach.
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Differences between SP and MP Surgical Approaches for TKA
Although rising from a seated position, both SP and MP groups seem to have weakened quadriceps function as well as deficits to hip abduction and external rotation that persist at least 6 months after TKA. Knee extension moments and knee power were both decreased after TKA compared with controls, with no significant differences seen between the MP and SP groups (►Figs. 2, 3). A similar situation was found at the hip where abduction and external rotation moments were decreased in both groups compared with controls, but not when compared with each other. During the preparatory phase of the STSU movement, the SP subjects had increased hip flexion at the earliest time point, which was no longer evident at 6 months. When combining this decreased hip flexion with the decreased vertical GRF and decreased knee flexion, it seems that the SP group may be unloading the affected limb more so than the MP group early in the recovery period. Although most of these hip and knee parameters did seem to move toward control levels earlier in the MP group, there were no significant differences found when the MP group was compared with the SP group, postoperatively. Reanalysis of these knee and hip parameters at a time point later than 6 months may provide more information as to whether the MP group truly has an advantage. Looking at the sitting movement, similar trends were seen, in which the MP group seemed to move toward control levels earlier in the postoperative period, but it remained statistically indistinguishable from the SP group. In particular, knee power was significantly lower than controls for the SP group 4 months post-TKA, whereas it was only significant for the MP group at 2 months post-TKA. Similarly, knee extension moment became insignificant by month 4 for the MP group and remained significantly lower than controls for the SP group at 6 months after TKA (►Fig. 2). At the hip, external rotation and abduction moments were significant for both TKA groups compared with controls at 2 months, but only the SP group retained these differences at 6 months. Although these findings once again allude to earlier functional recovery in the MP group, it’s important to keep in mind that the MP group was not significantly different from the SP group in these parameters postoperatively. Very few significant differences in any task parameters were seen at the 4- and 6-month time points as opposed to the other time points analyzed. Although this is expected as patients regain function after TKA, this may also highlight a limitation of this study that not all subjects initially enrolled were able to be included in analysis in both STSD and STSU tasks at each postsurgical evaluation. Interestingly, several parameters were significant at the 2- and 6-month time points, but not at the 4-month time point; however, significance at 4 months may have been affected by the increased absences in combination with subjects who did not complete the tasks as instructed. Although this was more of an issue for the MP group as opposed to the SP group, descriptive statistics did not show any bias toward one group over another in this respect. Chair height is known to have an effect on the ability of an individual to rise from a chair, particularly when below knee height.28,41 For this reason, the armless stool used in this
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study was adjusted to approximately 90 degrees of knee flexion for each subject. Foot position was not constrained as has been done in other studies and may have had some impact on rotation and abduction angles. This was done to allow subjects to situate themselves as they would on a daily basis, thus reflecting more realistic kinematic and kinetic compensation strategies used by these individuals to sit and stand. To reduce variability, the middle three trials of each subject were used when possible, often eliminating the first trial, in which some subjects may have still been getting accustomed to the STSU and STSD testing procedure. There may be functional differences that arise in other tasks which cannot be identified by this study alone. It is possible that patients are still adjusting their gait 6 months post-TKA and therefore a 1- to 2-year analysis may identify differences that are truly lasting. Knee power and knee flexion moments in particular are still significantly lower 6 months after surgery, indicating the need for a longer follow-up time to identify whether or not these parameters continue to show a deficit compared with controls and if there is any difference in outcome between SP and MP approaches. Although minimally invasive TKA may have some benefits in the early recovery period, the results of this study further support the claim that there is little benefit in functional outcome in the early and mid-term recovery period. It does not seem from the data presented in this study that the MP approach to TKA has a clear advantage over the SP approach.
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