The number of patients requiring knee replacements is increasing every year due to the combination of an aging population, increased patient expectancy, and rising levels of obesity. Therefore, newer implants, or design features, are introduced on a regular basis to try to improve patient outcomes. In 2013, DePuy Synthes launched the Attune Knee System to provide improved range of motion and address the unstable feeling some patients experience whilst performing everyday activities, such as going down stairs. The Attune cementless rotating platform knee was first implanted in September 2016. In 2004, the first Triathlon total knee designed by Stryker was implanted. Registry figures for both the Attune and Triathlon knees are promising with good implant longevity and outcomes.

There is only a small amount of literature available on the Attune knee as it is still a relatively new implant. Worldwide, the Triathlon is widely used and is also a well-established, successful implant used in the investigators' unit. This study will help to determine whether the Attune can improve patient outcomes in terms of gait analysis assessed by walking on a treadmill, patient reported outcome measures and X-ray outcomes when compared to the Triathlon knee. 90 patients who will undergo knee replacement and meet the inclusion criteria and agree to take part will be randomly placed in one of two groups to receive either the Attune or Triathlon knee implant.

The number of patients requiring Total Knee Arthroplasty (TKA) is increasing every year due to the combination of an aging population, increased patient expectancy, and rising levels of obesity which accelerates the onset of osteoarthritis. Therefore, newer implants, or design features, are introduced on a regular basis in an attempt to incrementally improve patient outcomes. In 2013, DePuy Synthes launched the Attune Knee System to provide improved range of motion (ROM) and address the unstable feeling some patients experience whilst performing everyday activities, such as descending stairs. The Attune Knee System features a gradually reducing femoral radius, an innovative s-curve design of the posteriorly stabilised cam, a tibial base which can be downsized or upsized two sizes versus the insert, novel patella tracking, lighter innovative instruments, and a new polyethylene formulation. These implant properties could potentially produce improved gait and speed, better stability of the knee in deep flexion, reduced joint forces, better patella tracking, improved operative flexibility and efficiency, and implant longevity. The system currently comprises 14 femoral sizes, 10 tibial sizes and 5 patella options.

Registry figures regarding the Attune Knee System are promising. The 2020 figures from the National Joint Registry (NJR) for England, Wales, Northern Ireland and the Isle of Man, show that the cumulative revision rate for the Attune knee (fixed bearing) was 2.7% (95% confidence interval (CI), 2.32-3.17) at 5 years (i.e. implant survivorship at 5 years of 97.3%) out of 25,723 knee joints. The cumulative revision rate for the Attune knee (rotating platform (RP)) was 1.7% (95% CI, 1.17-2.50) at 5 years (i.e. implant survivorship at 5 years of 98.3%) out of 4,254 knee joints. Figures from the Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR) 2020 Report show that the 5-year cumulative revision rate of the Attune Cruciate Retaining implant was 3.0% (95% CI, 2.7 to 3.5) out of 15,300 knees. The 5-year cumulative revision rate of the Attune Posterior Stabilized (PS) implant was 2.6% (95% CI, 2.1 to 3.3) out of 7,179 knees.

The Attune cementless Rotating Platform was first implanted in September 2016 and by February of 2021 over 22,000 have been implanted worldwide. There are only a small number of studies regarding the Attune implant due to its infancy. In 2004, the first Triathlon total knee (Stryker) was implanted. The Triathlon knee was designed to address the main reasons for revision surgery such as instability, patellofemoral tracking complications and loosening/osteolysis. There are cruciate-retaining, condylar-stabilising, posterior-stabilising and difficult primary options available. The single radius design allows for mid-flexion stability. Over 3 million Triathlon knees have been implanted worldwide.

Joint registries show a high rate of survivorship of the Triathlon with over 10 years of follow up. The 2020 figures from the NJR for England, Wales, Northern Ireland and the Isle of Man, show that the cumulative revision rate for the Triathlon was 2.2% (95% CI, 2.10-2.20) at 5 years (i.e. implant survivorship at 5 years of 97.8%) and 3.4% (95% CI, 3.18-3.56) at 10 years (i.e. implant survivorship at 10 years of 96.6%) out of 133,729 knee joints. Figures from the AOANJRR 2020 Report show that the cumulative revision rate of the Triathlon CR implant was 2.5% (95% CI, 2.4 to 2.7) at 5 years and 3.9% (95% CI, 3.7 to 4.2) at 10 years out of a total 50,402 knees. The cumulative revision rate of the Triathlon PS implant was 4.0% (95% CI, 3.6 to 4.5) at 5 years and 6.1% (95% CI, 5.5 to 6.9) at 10 years out of a total 8,755 knees.

To the investigators' knowledge, the only study to compare the Attune and Triathlon TKAs was a retrospective cohort analysis in 2018 using cemented components. The Attune implant was received by 1,178 patients, whilst the Triathlon implant was received by 5,707 patients. Patients who received the Attune TKA had a statistically significantly shorter length of stay and operating room time, were statistically significantly less likely to be discharged to a skilled nursing facility or other inpatient facility, and had statistically significantly lower total hospital cost than those who received the Triathlon implant.

To date, the large majority of studies comparing the Attune TKA to other implants have used the PFC Sigma TKA as a comparator. Most of these studies have examined cemented implants only. A retrospective review comparing 114 PFC Sigma cemented and 103 Attune cemented TKAs with a mean follow-up of 3.2 years found similar rates of patellar crepitus clunk and anterior knee pain. There were also no clinically significant differences in ROM, pain, or Knee Society Score (KSS) between the two groups. Another retrospective review compared migration of the cemented Attune fixed bearing CR tibial component with the cemented PFC-sigma fixed bearing CR tibial component. The overall migration at two years of both groups (38 Attune and 36 PFC Sigma) was comparable: mean 1.13 mm (95% CI, 0.97 to 1.30) for the Attune and 1.16 mm (95% CI, 0.99 to 1.35) for the PFC-sigma. At two years, the mean backward tilting was -0.43° (95% CI, -0.65 to -0.21) for the Attune and 0.08° (95% CI -0.16 to 0.31) for the PFC-sigma. The clinical outcomes and Patient Reported Outcome Measures (PROMs) (Knee injury and Osteoarthritis Outcome Score (KOOS), pain scores, KSS and Oxford Knee Score (OKS)) improved between pre-operation and two years post-operation and were not significantly different between groups. Radiolucent lines (RLLs) at the implant-cement interface were mainly seen below the medial tibial baseplate in 17% of the Attune patients and 3% of the PFC-sigma patients at two weeks (no significant difference), and at two years 42% and 9% of patients respectively (p=0.001). All implant-cement interface radiolucencies were less than 2 mm. It was noted that the version of the Attune tibial component examined in this study had since undergone modification by the manufacturer. In a further randomised controlled trial of 80 cemented Attune and 78 cemented PFC Sigma TKAs, there were no significant differences found in post-operative KSS, Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), ROM or Visual Analogue Scale (VAS) pain score between the two groups at 2-year follow-up. Both groups showed significantly improved outcomes 2 years after surgery.

Thirty TKA patients (15 Attune and 15 Sigma both CR FB) with a KOOS>70 and at least 1 year post-operation were assessed during five complete cycles of level walking, stair descent (0.18-m steps), deep knee bend, and sitting down onto and standing up from a chair, using a moving fluoroscope (25 Hz, 1 ms shutter time). Kinematic data were extracted by 2D/3D image registration. The results demonstrated similar tibiofemoral ranges of motion for flexion-extension, abduction-adduction, internal-external rotation, and anteroposterior (AP) translation for both groups. The pattern of AP translation-flexion-coupling differed between the two groups. The subjects with the Sigma TKA showed a sudden change in direction of AP translation around 30º of flexion, which was not present in the subjects with the Attune patients.

Musgrave Park Hospital (MPH) is one of the largest Orthopaedic centres in the United Kingdom (UK) performing more than 1,000 primary TKAs annually. It is part of the Belfast Health and Social Care Trust (BHSCT) in Northern Ireland. Nearly 1,000 Attune TKAs have been performed in MPH since 2022, with over 1,000 cementless Triathlons in more recent years.

MPH has a Compact Tandem Force-Sensing Treadmill (DBCEEWI-CE, AMTI Force and Motion, Watertown, MA, USA) capable of measuring vertical, anteroposterior and mediolateral forces and moments. It can measure 6 ground reaction force components (Fx,Fy,Fz, Mx,My,Mz), with a variable speed of 0-18 Km/h, linearity of <±0.5% full scale output, hysteresis of <±0.5% full scale output and maximum inclination 25% grade. This study will be in collaboration with Professor Justin Cobb's research team in Imperial College, London who have established expertise in this form of gait analysis. The Total or Partial Knee Arthroplasty Trial (TOPKAT) randomised controlled trial which included 264 total and 264 partial knees reported no significant differences in OKS between the two groups. At MPH, a subgroup of these patients (16 total knees, 11 partial knees and 16 volunteers with no knee replacements) completed post-operative gait analysis at a mean of 4.5 years following surgery. Analysis of this data showed no differences in gait symmetry between the two groups during level, downhill and uphill walking. Both groups demonstrated similar gait profiles during the three walking conditions to that of the healthy volunteers.

Design features of Attune vs Triathlon There are two key design features which are different between the Attune and Triathlon TKAs.

1. Coronal stability due to sagittal kinematics design. The Attune knee has a Gradually-Reducing radius of curvature termed the Attune Gradius curve from 5-65º flexion. The Triathlon has a Single Radius design from 10-110º flexion.
2. Sagittal (AP stability) variation due to Triathlon's 'Rotatory Arc' which attempts to achieve some rotational freedom to compensate for the Single Radius design but comes at the expense of increased AP laxity.

Rationale for the Study: To date, there has been a paucity of literature on the Attune TKA as it is still a relatively new implant. Worldwide, the Triathlon is widely used and is also a well-established, successful implant used in the investigators' unit. The single radius of the Triathlon may provide a less stable knee joint compared to the gradually reducing curve of the Attune knee which provides AP stability and greater ROM. The Triathlon TKA has approximately 5.6 mm of AP laxity but the literature suggests >10 mm of AP laxity is associated with a reduction in functional outcome scores. Therefore, the primary outcome of gait analysis is being used in this study as a more sensitive measure of knee function. Both the Attune and Triathlon TKAs have been designed with a similar evolution of the trochlear geometry - both are asymmetric laterally. This study will be able to determine whether the Attune can improve patient outcomes in terms of gait analysis, patient reported outcome measures and radiological outcomes. Based on previous work from Professor Justin Cobb's research team in Imperial College, London, it is postulated that gait analysis assessed on a decline will demonstrate a difference between the aforementioned Attune and Triathlon design philosophies.

Inclusion Criteria:

• Patients aged <65 years old who require a primary TKA for medial compartment osteoarthritis with a varus deformity.
• On examination patients must have a pain free ROM of both hips, both ankles and the opposite knee. A history of mild pain in any of these lower limb joints that does not inhibit mobility is acceptable.
• Must be able to walk at a pre-operative speed of at least 4 km/h (approximately 30 metres in 30 seconds).
• Must be able to walk outside unaided or with no more than a walking stick.
• Must be able to complete pre- and post-operative gait analysis.
• Participants must have a functional level of spoken and written English and must have a smartphone or have access to a smartphone to complete the online questionnaires on REDCap and to take photographs for virtual assessment of ROM.

Exclusion Criteria:

• Patients with valgus deformity of the knee.
• Patients with previous lower limb arthroplasty.
• Patients with previous lower limb open reduction and internal fixation (ORIF).
• Patients who are American Society of Anaesthesiologists (ASA) grade 3 or higher.
• Patients who are unable to adhere to the trial protocol (due to cognitive impairment, cannot speak English or for any other reason).