Summary

Knee laxity and some kinematic parameters of gait are restored in patients with a Nexgen total knee arthroplasty.

Abstract

Objective

Total knee arthroplasty (TKA) aims to alleviate pain and restore joint biomechanics to an equivalent degree to age-matched peers.

Zimmer Biomet’s Nexgen TKA was the most common implant in the UK between 2003 and 2016. This study compared the biomechanical outcomes of the Nexgen implant against a cohort of healthy older adults to determine whether knee biomechanics is restored post-TKA.

Methods

Patients with a primary Nexgen TKA and healthy adults over 55 years old with no musculoskeletal deficits or diagnosis of arthritis were recruited locally. Eligible participants attended one research appointment. Bilateral knee range of motion (RoM) was assessed with a goniometer. A motorised arthrometer (GENOUROB) was then used to quantify the anterior-posterior laxity of each knee. Finally, kinematic and spatiotemporal parameters of gait were analysed on a treadmill. An 8-camera Vicon motion capture system was used to generate the biomechanical model and process the data.
Preliminary statistical analyses were performed in SPSS (a = 0.05; required sample size for ongoing study: n=21 per group). All data was tested for normality and variance. The appropriate analyses were then performed depending on the results of these tests and on whether the samples were dependent or independent of each other.

Results

Twenty-one patients and 20 healthy older adults have been recruited into this study to date. The patient cohort was older (71.1±7.4 vs 62.6±4.6 years; p < 0.001, two-sample t-test) and had a greater BMI than the comparative group (30.8±4.8 vs 25.9±3.8kg/m2; p = 0.001).
The active knee RoM reported in healthy older adults’ knees were 136.8±9.1° and 135.3±9.5° (p = 0.10, paired t-test). Both were significantly greater than the RoM of the patients’ operated knees: 118.0±11.9° (p < 0.001, two-sample t-tests). The patients’ contralateral knee also had significantly poorer RoM when compared to the control group: 116.2±10.6° (p < 0.001, two-sample t-tests).
However, there were no inter- or intra-group differences in knee laxity (p > 0.05, paired and two-sample t-tests). Patients’ knees were found to translate anteriorly by 4.8±2.4mm (operative) and 5.4±2.5mm (contralateral). The knees of healthy older adults translated by 4.8±1.6mm (right) and 4.7±1.7mm (left).
The healthy older adults walked faster on level ground and downhill than the patient cohort (3.03±0.84km/h vs 2.50±0.56km/h; p = 0.02 & 2.82±0.87km/h vs 2.35±0.54km/h; p = 0.02, respectively). However, gait kinematics were comparable in the sagittal plane during stance phase. Peak knee flexion during swing phase was also comparable in the patient’s operative knee (52.2±10.5° vs 54.0±7.4° and 57.8±10.0°, p<0.05, two-sample t-tests). The contralateral knee had a poorer peak knee flexion than the controls, however (47.0±8.3°, p = 0.01 and p = 0.002).

Conclusions

Preliminary results suggest that knee laxity and some kinematic parameters of gait are restored in Nexgen TKA patients. However, knee RoM remained significantly poorer in the patient cohort post-operatively. Nonetheless, an average RoM of >110° was achieved by the patients. This suggests the implant provides sufficient RoM for most activities of daily living. Further improvements to knee kinematics and spatio-temporal parameters of gait may necessitate additional rehabilitation.