Evaluation of movement patterns in the unstable total knee replacement

Abstract

Total knee arthroplasty (TKA) is a highly effective treatment for reducing pain and improving function in end-stage osteoarthritis patients. Although patient satisfaction and implant survivorship rates are high, post-operative outcomes can vary significantly. In some cases where primary TKA fails, a revision operation is required. Instability of the knee accounts for approximately 20% of revision total knee arthroplasty (RTKA) surgeries worldwide, however this mode of failure remains poorly defined. Current methods for diagnosing instability are subjective and dependent on clinician expertise and patient perception. With the projected increase in TKA (and RTKA) volume, it is crucial that this mode of failure is better understood to inform diagnosis and subsequent course of management. It was proposed that objective tools should be applied to investigate biomechanical patterns to investigate functional characteristics underpinning TKA instability.

The aim of this thesis was to investigate whether movement characteristics associated with TKA instability could be detected and quantified within a clinical setting. Using biomechanical tools, this thesis sought to compare functional patterns in unstable TKA patients (TKA instability), well-functioning TKA patients and healthy controls (no TKA). The overarching hypothesis was that the unstable group would exhibit different (dysfunctional) movement patterns compared to the other two groups.

A total of 36 participants (21 unilateral TKA patients: TKA instability = 18; well-functioning TKA = 3; healthy control = 15) were examined to compare biomechanical patterns during functional tasks across a number of explorative studies. The 18 patients with TKA instability were examined 2-5 years post-operation. The unilateral well-functioning TKA patients were examined at least 1 year after surgery.

Preliminary work examined whether a pressure mat could be used to detect movement characteristics within a typical outpatient clinical setting. This study measured limb loading during simple functional tasks. Bilateral stance and level walking gait were examined in patients with suspected TKA instability (n = 10) and in a convenient sample of healthy controls (n = 10). Pressure distribution was analysed at 10-second intervals in the final minute of bilateral stance. Walking gait was divided into three phases – heel strike, mid-foot stance and toe-off – for time and pressure analyses, as well as overall foot contact time. Limb loading discrepancy was compared between limbs (within groups) and across groups. Pressure analyses showed clear differences in load distribution between limbs in the unstable group. This was most obvious throughout bilateral stance, but also when walking, where patients tended to offload the operated side. This group also performed each phase of gait slower and had longer overall contact time compared to healthy controls.

Initial findings showed that pressure mat analyses were able to demonstrate subtle differences in loading strategies within a clinical environment. As such, this tool was deemed useful for evaluating loading parameters. Pressure analyses were subsequently recommended during more challenging functional tasks.

The second developmental study assessed pressure distribution during a step-down task as unstable TKA patients frequently complain about difficulty walking downhill or descending stairs. Fourteen participants (unstable TKA = 7, well-functioning TKA = 3, healthy control = 4) were examined. All descended four steps per trial onto a pressure mat, alternating which foot made contact with it. Phases of step-down were divided similarly to level walking gait (above). Initial analyses showed that the automated time-pressure gait algorithm used by the data collection software was not applicable to the step-down task. Consequently, the analysis protocol was adapted to focus only on pressure loading. This showed that toe and mid-foot contact phases (during unavoidable weight acceptance) were indifferent between groups, but unstable TKA patients exhibited increased loading disparities and variation during arch and heel contact. However, the modified analysis was insufficient for demonstrating significant differences in mechanical patterns. It was determined that the pressure mat remains a useful tool but perhaps a more constrained task that challenges a greater range of motion (ROM) would yield clearer differences in biomechanical patterns.

Sixteen participants (unstable TKA = 8, well-functioning TKA = 3, healthy control = 5) were examined across bilateral stance, level walking gait and five-times sit-to-stand (5-STS) from a chair. Time and pressure parameters were examined as outlined above for within- and between-group analyses. Each 5-STS trial was split into individual 1-STS repetitions. Each individual 1-STS repetition was further sub-divided into ascent, stance, and descent phases. The key sit-to-stand assessment time variables were total 5-STS time, 1-STS, ascent, time to peak force during ascent and stance duration (between ascent and descent). Key pressure variables were peak pressure during ascent, rate of peak pressure development, side-to-side limb pressure distribution during stance and ‘Shoogle Factor’, which is a surrogate measure of dynamic postural stability.

Bilateral stance showed greater limb loading discrepancy (clear offloading of the operated side) in both TKA groups compared to healthy controls. Unstable TKA patients showed greater limb loading variation and longer contact times than the other two groups during gait examination. Peak limb loading pressure was significantly different between groups during the toe-off phase of gait, which is a surrogate measure of propulsive power.

Limb loading during 5-STS proved a highly effective test for demonstrating differences in functional loading patterns between groups. Those with TKA instability performed 1-STS significantly slower than the other two groups and had greatest variation in stance time. This group also showed greatest limb loading asymmetry in peak pressure and rate of peak pressure development during ascent. Unstable TKA patients tended to shift their weight away from the operated limb. Interestingly, analysis of total pressure produced (by both limbs combined) during ascent showed that both groups of TKA patients exhibited substantially lower peak pressure expression compared to healthy controls. However, as all TKA patients were able to complete the task, it was suggested that they may develop compensatory strategies that warrant further investigation. This study deemed that 5-STS was an effective test for assessing biomechanical patterns associated with TKA instability and showed clearer differences in movement strategies compared to well-functioning TKA patients and healthy controls.

An existing study examining satisfied TKA patients with varying degrees of knee joint laxity suggested that some patients are able to develop coping mechanisms to stabilise the knee, whereas others do not (Hamilton et al., 2020). Thus, the concept of “secondary (muscular) stabilisers” was proposed for controlling movement about the knee. Therefore, electromyographic (EMG) analysis was applied to lower limb muscles (Rectus Femoris, Vastus Medialis and medial hamstrings) during 5-STS. Proportional activation (signifying how long a muscle was active for) was reported relative to total 5-STS completion time. Muscle activation patterns showed substantially increased proportional activation of the hamstrings in unstable TKA patients compared to well-functioning TKA patients and healthy controls. TKA groups also exhibited increased Rectus Femoris proportional activation on the operated limb compared to the un-operated side. Elevated quadriceps-hamstrings co-activation observed in TKA groups was suggested to be a protective mechanism on the operated limb to cope with functional task execution.

Clinical functional outcome tests demonstrated that, overall, unstable TKA patients performed timed tasks slower than the other two groups, which indicates impaired function. Range of motion and joint position sense were similar in both TKA groups but impaired in comparison to healthy controls. Isometric quadriceps and hamstrings strength were weaker in both TKA groups, but unstable TKA patients showed significantly reduced strength measurements compared to controls. Power output during unilateral leg extension was markedly lower in the TKA instability group, particularly on the operated leg. For both strength and power measurements well-functioning TKA patients showed higher scores than those with TKA instability. Collectively, these findings support the existence of a spectrum of recovery and function in the TKA population following surgery.

This was an explorative thesis that aimed to evaluate movement patterns associated with TKA instability. Findings conclude that it is possible to apply objective tools within an outpatient clinical environment to quantify function. Assessment of pressure distribution during 5-STS proved the most effective task for differentiating dynamic functional patterns under load in unstable TKA patients. EMG analyses were also beneficial for discerning differences in muscle activation patterns in unstable TKA patients, which demonstrated that altered muscular involvement in force production and limb and stabilisation exist during dynamic functional activities.

Overall, the large variation in functional outcomes supports the proposition that a spectrum of function within the TKA population. This thesis found that functional patterns are detectable and quantifiable with the use of objective tools. Further and more detailed investigation into biomechanical patterns is warranted to better understand the variation in functional ability within the TKA population, but particularly so within unstable TKA patients. Clarification of functional patterns associated with TKA instability may help inform future clinical practice.