Bi-Unicompartmental Knee Arthroplasty — Do Patients Become More Active?

Preserving both cruciate ligaments and more native bone did not translate into greater everyday physical activity after robotic bi-unicompartmental knee arthroplasty than after total knee arthroplasty. This secondary analysis of a randomised trial used accelerometers for up to two years. The result is clinically interesting, but the small sample does not establish equivalence between the procedures.

Polish version · For Doctors and Physiotherapists

Why is this study interesting?

Bi-unicompartmental knee arthroplasty replaces the medial and lateral tibiofemoral compartments with two separate unicompartmental implants. It preserves the cruciate ligaments and more native structures than conventional total knee arthroplasty. Biomechanically, that is attractive, but the clinically relevant question is whether the patient actually walks more and becomes more active in everyday life.

The study was published on 10 September 2026 in Bone & Joint Research.

Question and methods

This was an analysis of secondary outcomes from the TRUCK randomised trial. Patients were allocated to manual total knee arthroplasty or robotic arm-assisted bi-unicompartmental knee arthroplasty. Sixty-nine participants had usable accelerometer data for this analysis: 38 after total knee arthroplasty and 31 after bi-unicompartmental arthroplasty.

A triaxial accelerometer was worn on the operated thigh for seven days, with at least three valid recording days required for analysis. Measurements were obtained before surgery and at one and two years.

• total activity-related energy expenditure;
• time spent in moderate-to-vigorous physical activity;
• time spent in light activity;
• sedentary time;
• the relationship between objectively measured activity and patient-reported outcome measures.

The trial involved a selected population with disease in the medial and lateral compartments, preserved cruciate and collateral ligaments, no major deformity and no advanced patellofemoral disease.

Results

No statistically significant between-group differences were found in any objectively measured activity parameter at either one or two years. This included total activity energy expenditure, moderate-to-vigorous activity, light activity and sedentary time.

At one year, the bi-unicompartmental group showed numerically greater total activity and somewhat more moderate-to-vigorous movement, but the differences were not statistically significant and were not maintained at two years.

Preoperative activity was a stronger predictor of postoperative activity. Higher baseline energy expenditure and more moderate-to-vigorous activity were associated with greater activity after surgery.

Both groups improved similarly on clinical questionnaires. However, the association between patient-reported outcomes and objectively measured movement was weak and inconsistent. A patient may therefore report substantial improvement in pain and function without becoming more active in daily life.

Clinical interpretation for an orthopaedic surgeon

Tissue preservation and more physiological knee kinematics do not guarantee a change in real-world behaviour. Postoperative activity is also shaped by habits, preoperative activity, body mass, comorbidities, patient goals, environment and rehabilitation.

The study also illustrates why questionnaires and accelerometry should not be treated as interchangeable outcomes. Questionnaires assess symptoms and perceived capability; accelerometers record how much movement the patient actually undertakes. These tools answer different questions.

Limitations

• Physical activity was a secondary outcome, and the trial was not powered specifically to detect differences in these measures.
• The sample was small and the number of valid recordings declined over time. At two years, accelerometer data were available for 24 patients after total knee arthroplasty and 15 after bi-unicompartmental arthroplasty.
• Three valid days of recording were sufficient for analysis and may not capture full week-to-week variability.
• The comparison was robotic bi-unicompartmental arthroplasty versus manual total knee arthroplasty, so the effects of implant strategy and robotic technology cannot be separated.
• The findings apply to carefully selected patients with preserved ligaments and a specific pattern of osteoarthritis.

What might change in practice?

When counselling patients, greater everyday activity should not be promised simply because a procedure preserves the cruciate ligaments and more native knee structures. It may be more useful to define realistic activity goals before surgery and to plan rehabilitation not only around regaining the ability to walk, but also around progressively changing physical-activity behaviour.

The study also supports complementing conventional clinical outcomes with real-world activity measures in trials of techniques that are intended to improve higher-level function.

What does the study not prove?

• It does not establish equivalence between the two operations; absence of statistical significance in a small, underpowered analysis is not evidence of identical effects.
• It does not exclude differences in maximal performance, kinematics, gait speed or other outcomes that this analysis was not designed to assess reliably.
• It does not justify broadening indications for bi-unicompartmental arthroplasty beyond patients who meet the trial criteria.
• It cannot separate the effect of robotic assistance from the effect of preserving ligaments and reducing the extent of bone resection.

Source

Kanakala S et al. Effect of robotic arm-assisted bi-unicompartmental knee arthroplasty versus total knee arthroplasty on objectively measured physical activity: secondary outcomes of a randomized controlled trial. Bone Joint Res. 2026;15(9):1136–1146. DOI: 10.1302/2046-3758.159.BJR-2026-0018.R1. PubMed.

Content prepared by: Tomasz Poboży - orthopaedic surgeon.