Unicompartmental Versus Total Knee Replacement for Medial Compartment Knee Osteoarthritis (FLEX-UNI)

September 21, 2026 updated by: Golden Jubilee National Hospital

A Functional, Clinical, and Biomechanical Examination of Unicompartmental Versus Total Knee Arthroplasty for Medial Compartment Knee Osteoarthritis (FLEX-UNI): A Randomized Controlled Trial

For patients with osteoarthritis affecting only the inner part of the knee, surgeons can offer either a partial knee replacement, which resurfaces the worn compartment alone, or a total knee replacement. Both are recommended treatments, but there is no agreement on when to choose one over the other. Partial knee replacement preserves the anterior cruciate ligament and more of the natural knee, and recovery is usually quicker, but it is used in only about one in ten knee replacements in the United Kingdom.

Studies comparing the two have mostly relied on patient questionnaires, which may not be sensitive enough to detect differences in how the knee actually functions. Studies that have measured movement directly have concentrated on level walking, which is not demanding enough to separate the two operations. Difficulty with stairs is one of the longest lasting problems patients report after knee replacement.

The FLEX-UNI trial will compare the two operations using three-dimensional motion analysis during more demanding everyday activities. Forty patients who are suitable for a partial knee replacement will be randomly allocated to receive either a partial or a total knee replacement. Patients and the researchers measuring their movement will not know which operation was performed. The main measurement is how far the operated knee bends while stepping down from a 20 cm step, measured one year after surgery. Movement during walking, stair climbing, slopes, sitting and standing, and balance will also be measured, along with questionnaires, x-rays, and complications. Results will be compared against measurements from people with healthy knees.

Study Overview

Status

Active, not recruiting

Conditions

Intervention / Treatment

Detailed Description

FLEX-UNI is a single-centre, prospective, randomized, double-blinded, controlled superiority trial comparing medial unicompartmental knee arthroplasty (UKA) with total knee arthroplasty (TKA) in patients with anteromedial knee osteoarthritis. Forty participants will be randomized 1:1. The trial is conducted at a high-volume elective arthroplasty centre and participants are followed for 12 months.

Participants are identified from the clinic lists of participating consultant orthopaedic surgeons and screened against the eligibility criteria by a member of the research team and by the treating consultant. Following written consent and baseline assessment, participants are randomized using sequentially numbered, opaque, sealed envelopes prepared and opened by a member of the research team not involved in screening, recruitment, or outcome assessment.

Participants allocated to UKA receive a cemented medial fixed-bearing Persona Partial Knee (Zimmer Biomet, Warsaw, Indiana, USA), implanted using a spacer block technique that resurfaces the medial compartment without deliberate correction of limb alignment. Participants allocated to TKA receive a cemented Triathlon cruciate-retaining total knee system without patellar resurfacing (Stryker, Mahwah, New Jersey, USA), implanted using a measured resection technique with a mechanical alignment target. Both procedures use conventional manual instrumentation, without robotic assistance or computer navigation, and are performed by the same group of consultant surgeons, each of whom performs both operations.

Suitability for medial UKA is confirmed intraoperatively at arthrotomy in all participants, including those allocated to TKA. Participants allocated to UKA who are found at arthrotomy to be unsuitable, principally because of disease progression beyond anteromedial osteoarthritis or a deficient anterior cruciate ligament, undergo TKA.

Participants are blinded to allocation. A standard-size sterile dressing is applied to the surgical wound by an independent healthcare support worker before biomechanical testing to conceal scar length, and participants are not shown their postoperative radiographs unless there is a clinical need. Research staff performing biomechanical assessment, outcome scoring, and statistical analysis are blinded to allocation, with unblinding only after analysis is complete. The treating surgeon cannot be blinded.

Assessments take place preoperatively, immediately postoperatively, and at six weeks, six months, and 12 months. Biomechanical assessment is performed in the centre's Motion Analysis Laboratory using a Vicon motion capture system (Vicon Motion Systems Ltd, Yarnton, UK) and two AMTI force plates (Advanced Mechanical Technology Inc., Watertown, Massachusetts, USA). Tasks are level walking, incline walking, decline walking, step ascent, step descent, sit-to-stand-to-sit, and single-leg balance.

A matched healthy control group is drawn from an existing motion analysis database held at the study centre, comprising individuals without known knee pathology previously assessed using an identical protocol under separate ethical approval. Control data do not contribute to the primary comparison between arms and are used only to contextualize findings against values expected in a healthy population. Healthy controls are not prospectively enrolled in this trial.

The sample size was calculated for the primary outcome, assuming 95 degrees for the UKA group and 89 degrees for the TKA group with a pooled standard deviation of 5.5 degrees, two-sided alpha of 0.05, and 80% power, giving 15 participants per group for an analysis of covariance with baseline as a covariate. Allowing for an estimated 10% intraoperative conversion from UKA to TKA and 15% loss to follow-up, the recruitment target is 20 per group. The trial is powered for the primary outcome alone, and all secondary outcomes are exploratory.

The primary analysis is performed on the per-protocol population, defined as participants who received their randomized implant and completed the 12-month biomechanical assessment. Participants found intraoperatively unsuitable for UKA at arthrotomy are excluded from the primary analysis regardless of allocation.

The trial is funded by Zimmer GmbH (Winterthur, Switzerland).

Study Type

Interventional

Enrollment (Estimated)

40

Phase

  • Not Applicable

Contacts and Locations

This section provides the contact details for those conducting the study, and information on where this study is being conducted.

Study Locations

  • United Kingdom
    • West Dunbartonshire
      • Clydebank, West Dunbartonshire, United Kingdom, G81 4DY
        • Golden Jubilee National Hospital

Participation Criteria

Researchers look for people who fit a certain description, called eligibility criteria. Some examples of these criteria are a person's general health condition or prior treatments.

Eligibility Criteria

Ages Eligible for Study

18 years to 75 years (Adult, Older Adult)

Accepts Healthy Volunteers

No

Genders Eligible for Study

All

Description

Inclusion Criteria:

  • Suitable for medial unicompartmental knee arthroplasty
  • Aged between 18 and 75 years inclusive
  • Taller than 1.5m in height (the 10th percentile for the population)
  • Functionally intact anterior cruciate ligament
  • Able to give informed consent
  • Able to return to the study centre for follow-up
  • BMI under 40

Exclusion Criteria:

  • Inflammatory joint disease e.g. rheumatoid arthritis
  • Previous hip procedure within the previous twelve months
  • Previous knee surgery other than arthroscopy and medial menisectomy
  • Previous ankle surgery
  • Symptomatic foot, hip or spinal pathology
  • Any medical condition limiting normal ambulation e.g. stroke, Charcot-Marie-Tooth disease, severe lymphedema, advanced cardiac disease
  • Unable to read and understand English and follow verbal and visual instructions unless they can bring a relative or friend to all study assessments to act as a translator

Study Plan

This section provides details of the study plan, including how the study is designed and what the study is measuring.

How is the study designed?

Design Details

  • Primary Purpose: Treatment
  • Allocation: Randomized
  • Interventional Model: Parallel Assignment
  • Masking: Double

Arms and Interventions

Participant Group / Arm
Intervention / Treatment
Experimental: Unicompartmental knee arthroplasty (UKA)

Description: Cemented medial fixed-bearing unicompartmental knee arthroplasty, implanted using a spacer block technique that resurfaces the medial compartment without deliberate correction of limb alignment, using conventional manual instrumentation
Device: Persona Partial Knee (PPK)

Cemented medial fixed-bearing unicompartmental knee arthroplasty using the Persona Partial Knee (PPK; Zimmer Biomet, Warsaw, Indiana, USA).
Active Comparator: Total knee arthroplasty (TKA)

Cemented cruciate-retaining total knee arthroplasty without patellar resurfacing, implanted using a measured resection technique with a mechanical alignment target, using conventional manual instrumentation. This is the standard total knee implant used at the study centre.
Device: Triathlon cruciate-retaining total knee system.

Cemented Triathlon cruciate-retaining total knee system without patellar resurfacing (Stryker, Mahwah, New Jersey, USA).

What is the study measuring?

Primary Outcome Measures

Outcome Measure
Measure Description
Time Frame
Maximum knee flexion of the operated knee during step descent
Time Frame: 12 months post-operatively

Peak flexion angle of the operated knee in the sagittal plane, in degrees, during descent from a 20 cm step with the operated limb acting as the trailing limb, measured using three-dimensional motion analysis.
12 months post-operatively

Secondary Outcome Measures

Outcome Measure
Measure Description
Time Frame
Maximum knee flexion during step descent, operated limb as trailing limb
Time Frame: 6 months post-operatively
6 months post-operatively
Knee kinematics and kinetics
Time Frame: baseline, 6 months, 12 months

Knee kinematics: sagittal, coronal, and transverse plane joint angles during level walking, incline walking, decline walking, step ascent, step descent, and sit-to-stand-to-sit.

Knee kinetics: knee joint moments and ground reaction forces during tasks performed on force plates.
baseline, 6 months, 12 months
Spatiotemporal and balance
Time Frame: baseline, 6 months, 12 months.

Spatiotemporal gait parameters: walking speed and step length. Single-leg balance: time to failure, in seconds.
baseline, 6 months, 12 months.
Patient satisfaction
Time Frame: 6 weeks, 6 months, 12 months

Patient satisfaction on a Fpur-point Likert scale.
6 weeks, 6 months, 12 months
Radiographic assessment
Time Frame: to 12 months postoperatively

11. Radiographic assessment of implant position, alignment, and signs of loosening or migration, using a modified Knee Society Roentgenographic Evaluation and Scoring System. No summary score is produced.
to 12 months postoperatively
Complications and adverse events
Time Frame: to 12 months postoperatively

12. Complications and adverse events, including deep and superficial infection, thromboembolic events, intraoperative fracture, neurovascular injury, wound complications, and any reoperation or revision.
to 12 months postoperatively
Implant survivorship
Time Frame: 12 months

Implant survivorship, reported as the number of implants in situ.
12 months
Intraoperative conversion from unicompartmental to total knee arthroplasty, reported as the number of participants.
Time Frame: day of surgery
day of surgery

Other Outcome Measures

Outcome Measure
Measure Description
Time Frame
Patient reported outcome (Oxford Knee Score)
Time Frame: baseline, 6 weeks, 6 months, 12 months.

Oxford Knee Score, 12 items, total score 0 to 48, higher scores indicate better outcome.
baseline, 6 weeks, 6 months, 12 months.
Patient reported outcome (EuroQual 5D-5L questionnaire)
Time Frame: baseline, 6 weeks, 6 months, 12 months.

9. EQ-5D-5L index value and visual analogue scale.
baseline, 6 weeks, 6 months, 12 months.
Patient reported outcome (Forgotten Joint Score)
Time Frame: 6 weeks, 6 months, 12 months.

Forgotten Joint Score-12, score 0 to 100, higher scores indicate greater ability to forget the operated joint.
6 weeks, 6 months, 12 months.
Patient reported outcome (University of California Los Angelees Activity Score)
Time Frame: baseline, 6 weeks, 6 months, 12 months.

8. UCLA Activity Score, single item, 1 to 10, higher scores indicate greater activity.
baseline, 6 weeks, 6 months, 12 months.

Collaborators and Investigators

This is where you will find people and organizations involved with this study.

Sponsor

Golden Jubilee National Hospital

Investigators

  • Principal Investigator: Nicholas Ohly, FRCS Ed, Golden Jubilee University National Hospital

Publications and helpful links

The person responsible for entering information about the study voluntarily provides these publications. These may be about anything related to the study.

General Publications

Study record dates

These dates track the progress of study record and summary results submissions to ClinicalTrials.gov. Study records and reported results are reviewed by the National Library of Medicine (NLM) to make sure they meet specific quality control standards before being posted on the public website.

Study Major Dates

Study Start (Actual)

April 26, 2021

Primary Completion (Estimated)

October 15, 2027

Study Completion (Estimated)

October 15, 2027

Study Registration Dates

First Submitted

July 17, 2019

First Submitted That Met QC Criteria

June 3, 2021

First Posted (Actual)

June 4, 2021

Study Record Updates

Last Update Posted (Actual)

September 24, 2026

Last Update Submitted That Met QC Criteria

September 21, 2026

Last Verified

September 1, 2026

More Information

Terms related to this study

Keywords

Additional Relevant MeSH Terms

Other Study ID Numbers

  • Ortho 18-03
  • 251071 (Other Identifier: Health Research Authority)
  • 19/SS/0124 (Other Identifier: Scotland B Research Ethics Committee)

Plan for Individual participant data (IPD)

Plan to Share Individual Participant Data (IPD)?

NO

Drug and device information, study documents

Studies a U.S. FDA-regulated drug product

No

Studies a U.S. FDA-regulated device product

No

This information was retrieved directly from the website clinicaltrials.gov without any changes. If you have any requests to change, remove or update your study details, please contact register@clinicaltrials.gov. As soon as a change is implemented on clinicaltrials.gov, this will be updated automatically on our website as well.

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