Longevity of the SAIPH® Medially Stabilised Knee
Other manufacturers have introduced a ball-and-socket asymmetric tibial constraint to platforms with a traditional ‘keel’ tibial design used on unconstrained bearing options. Some have also produced cementless component versions. Higher revision rates17,43, particularly tibial loosening44 have been associated with these combined design characteristics. The MRK™ and SAIPH® Knees include a different fixation design to traditional keel designs, which is intended to support rotational torque at the implant-bone interface.
The first clinical follow up on MRK™ patients who had received the implant from 1994 commented specifically on the matter and showed that the increased congruence of the asymmetric tibial bearing had not increased the rate of loosening15. From 15,921 procedures recorded by the NJR over 18.5 years, the MRK™ has been revised for aseptic loosening of the tibia significantly fewer times (p<0.001) than all other TKRs in the UK16.
The SAIPH® Knee features an optimised stem-and-pegs design with a stippled cement interlocking interface with additional anti-rotation fins. From radiographic analysis Katchky et al. found no progressive lucent lines, no non-progressive lucent lines >2mm and there were no incidences of osteolysis at 5 years follow-up30. Just one SAIPH® Knee has been revised in the UK for aseptic loosening of the tibia, which is below expectations based on all other TKRs in the NJR (p=0.282)45. Radiostereometric analysis (RSA) also indicates stability of the components46 and this is now supported by low revision rates >5 years follow-up17,43.
Survivorship in Outcomes Studies
In their independent 11-surgeon multicentre study of the SAIPH® Knee, from 408 SAIPH® Knees implanted, Baré et al. reported a survivorship of 97.8% at 2 years32. In their study of 225 patients, Walter et al. reported 98.7% survivorship at 2 years29. In their five-year outcomes study from two centres and 100 patients, Katchky et al. reported a 98% survivorship30. All studies reported comparable complications to other TKR populations29,30,32.
Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR)
In its 2021 Annual Report, the AOANJRR has sufficient data to present revision rates for some TKR devices up to 20 years post operation. The overall revision rate for all TKRs is 3.3% at 5 years, 4.8% at 10 years, 6.5% at 15 years and 8.1% at 20 years for indication of osteoarthritis, which accounts for 97.7% of procedures43.
A trend to increasing use of medially stabilised knees is reported by the AOANJRR used since 2012. In 2020 use of medially stabilised knees increased to 9.8% of all TKRs47. Given the recent rise in use, medially stabilised knees represent a shorter follow-up period and fewer knees than the traditional CR and PS knee types. Nevertheless, medially stabilised knees are demonstrating lower cumulative percent revision rates than other knee types in the longer term with 6.0% at 15 years 48.
When compared to other medially stabilised knee designs, the SAIPH® and MRK™ knees are reported with the lowest cumulative rate of revisions for primary diagnosis of OA with mean 2.2% at 5 years post operation (95% CI: 1.7-2.9) for the SAIPH® Knee (Figure 8), which compares favourably to the rate of revisions for all knees with primary diagnosis of OA at the same time point (3.3%) and is in line with the best-performing minimally stabilised knee designs49.
Figure 8: AOANJRR cumulative percent revision of primary total knee replacement with a medially stabilised design (primary
National Joint Registry (NJR)
Medially stabilised knees have been a regular feature of the NJR, despite not being recognised as a separate category. Over 25,000 medially stabilised knees have been recorded since the NJR started collecting data in 200317. The majority of those procedures used the MRK™. In its 2021 Annual Report including data up to 31 December 2020, the NJR includes 1,855 SAIPH® Knees and a cumulative revision rate of 1.42% at 5 years post operation (95% CI: 0.89-2.25), which is lower than the 5-year revision rate for the MRK™.
In the most recent Summary Data Report from the NJR on the SAIPH® Knee (February 2022), data is reported for 2,374 knees (2,103 patients) with mean implantation time of 3.1 years, maximum 12.1 years45. Overall 25 knees have been revised (1%) and the reported cumulative revision rate is 2.2% at 9 years post operation (95% CI: 1.0-4.9), which compares well to the 9-year cumulative revision rate of 3.0% (95% CI: 2.9-3.0) for all TKRs in the NJR45.
SAIPH® Clinical Rationale
SAIPH® Operative Technique
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Orthopaedic Data Evaluation Panel (ODEP)
ODEP provide a service evaluation for TKR ratings and award ratings based on ongoing assessment of implant performance (rate of revision), and the number of patients for which principally UK data is available.