Materials and methods

This prospective large cohort, nonrandomized, multicenter study was approved by the Human Research Ethics Committees at the St Vincent’s Hospital, Melbourne (HREC-D 151/15), The Avenue Hospital, Melbourne (HREC 194) and the Whangarei Hospital, Whangarei (Northland District Health Board 2015-29), with site approvals obtained from the participating hospitals. Between January 2016 and May 2020, 608 TKAs (568 patients) were performed by 15 surgeons in 16 sites using the SAIPH Knee (MatOrtho Ltd., Leatherhead, UK). The design of the prosthesis is characterized by asymmetry between compartments, with the medial side of the prosthesis a single radius, spherical component congruent with the medial tibial polyethylene component through a full range of motion (ROM). The lateral side is not congruent allowing anteroposterior translation in addition to rotation. Changes in the second generation of the prosthesis were confined to the nonarticular portion of the prosthesis to improve anatomical replication and bone preservation. Adult individuals requiring a primary knee arthroplasty were invited to participate in the study. Indications for the use of the prosthesis were determined by the treating surgeon and aligned to the manufacturer’s instructions for use. The exclusion criteria included pregnancy or planning on becoming pregnant during the follow-up period for women; individuals with local or systemic infection; severe neurological, vascular or muscular deficiencies; individuals with severe bone loss or collateral ligament instability. Invited patients were provided with the patient information sheet and consent form at the surgical booking consultation and provided an opportunity to ask questions of the surgeon. Patients returned a signed consent form at the time of booking or later by mail or at a subsequent consultation prior to surgery. Surgeons were selected on the basis that they were all established knee arthroplasty surgeons but had only recently started using this prosthesis. No surgeon was involved in the design or had any commercial interest in the prosthesis. All surgeons received a full introduction to the concept, the device and fundamental surgical principals prior to use. No additional training specific to the use of the prosthesis was required. Each surgeon had performed at least five cases with the prosthesis prior to patient recruitment. All devices were cemented, the posterior cruciate ligament (PCL) was fully excised in all cases and coronal balance in flexion and extension was achieved either through soft tissue release or bony alignment strategies depending upon the individual surgeon’s preference. Indeed, common variations in surgeons’ established practice, such as the threshold for patella resurfacing, were deemed to constitute an acceptable representation of general use.

Demographic data included age, gender, side, date of operation, body mass index, indication for surgery, and follow-up length. The cohort demographics, indications for surgery, application of alignment strategy, and patellar resurfacing were indicative of standard TKA practice in the participating hospitals (Table 1). PROMs included the Knee injury and Osteoarthritis Outcome Score (KOOS), the Oxford Knee Score (OKS), the EuroQol 5-Dimension visual analog scale (VAS), the University of California Los Angeles activity score, the Forgotten Joint Score (FJS), and patient satisfaction scores. Patients were asked “Overall, how are your problems now, compared to before your operation?” with a 5-point Likert response. Patients were also asked “How would you describe the results of your operation?” on a 5-point Likert response as previously described [14], as well as a 100-mm VAS (with “least satisfied” at the left end and “most satisfied” at the right end of the line) that corresponded with how satisfied they were with their knee arthroplasty overall (Fig. 1). Patients were clinically evaluated preoperatively and at 6 weeks, 6, 12, and 24 months postoperatively, with any complications and all-cause revisions recorded at the time of reporting. The ROM and PROMs were collected preoperatively and at 12 and 24 months postoperatively.

Parameter | Value
Mean age ± SD (range) | 68 ± 8 (38-92)
Gender, male:female % (no.) | 49.8:50.2 (303:305)
Surgical side, left:right % (no.) | 43.8:56.3 (266:342)
Mean BMI ± SD, kg/m2 (range) | 31.4 ± 6.5 (15.4-78.5)
Mean follow-up ± SD, months (range) | 25.4 ± 2.2 (20.1-41.7)
Indication for surgery, % (no.) | 
Osteoarthritis | 95.6 (581)
Inflammatory arthritis | 2.0 (12)
Osteonecrosis | 0.8 (5)
Trauma | 0.5 (3)
Other | 0.8 (5)
Not specified | 0.3 (2)
Alignment, % (no.) | 
Mechanical alignment | 91.4 (556)
Modified kinematic alignment | 8.6 (52)
Instrumentation (alignment referencing), % (no.) | 
Intramedullary femur, extramedullary tibia | 42.7 (260)
All intramedullary | 29.8 (181)
Patient-specific (image derived instrumentation) | 21.4 (130)
Navigation | 4.9 (30)
Other | 0.2 (1)
Not specified | 1.0 (6)
Patella resurfacing, % (no.) | 
Resurfaced | 65.6 (399)
Not resurfaced | 34.0 (207)
Not specified | 0.4 (2)

Figure 1: Analog VAS satisfaction scale: patients were asked to mark the line at the position that best represents their level of satisfaction with their TKA.

Participating surgeons were required to perform uniform face-to-face consultations for patients preoperatively and at 6 weeks and 12 and 24 months postoperatively, in alignment with the study protocol. At each of these reviews, the surgeons collected PROMs and clinical data on a standardized form. All collected study information was then forwarded to a centralized data collection. All data were recorded onto the Socrates Orthopaedic Outcomes Software (Ortholink, PTY Ltd., Sydney, Australia).

Data analyses

Statistical analysis was performed using the GraphPad Prism software (version 9.3.1, December 2021). Datasets were assessed for normality using the Shapiro-Wilk test. Nonparametric Mann-Whitney or Kruskal-Wallis tests with post hoc Dunn’s multiple comparisons were used to compare ROM and PROMs data at each time point. Data are reported as mean ± standard deviation with 95% confidence interval (CI) or min-max range. The Kaplan-Meier method was used to evaluate implant survivorship. The threshold for significance for all statistical analyses was P < .05.