Roentgen Stereophotogrammetric Analysis (RSA) is the gold standard for measuring implant micromotion thereby predicting implant loosening. Early migration has been associated with the risk of long-term clinical failure. We used RSA to assess the stability of the Australian designed cementless hip stem (Paragon TM) and now report our 5-year results.

Fifty-three patients were prospectively and consecutively enrolled to receive a Paragon hip replacement. Tantalum beads were inserted into the bone as per RSA protocol and in the implant. RSA x-rays were taken at baseline 1–4 days post-surgery, at 6 weeks, 6 months, 12 months, 2 years, and 5 years. RSA was completed by an experienced, independent assessor.

We reported the 2-year results on 46 hips (ANZJS 91 (3) March 2021 p398) and now present the 5-year results on 27 hips. From the 2-year cohort 5 patients had died, 8 patients were uncontactable, 1 patient was too unwell to attend, 5 patients had relocated too far away and declined. At 5 years the mean axial subsidence of the stem was 0.66mm (0.05 to 2.96); the mean rotation into retroversion was 0.49˚ (−0.78˚ to 2.09˚), rotation of the stem into valgus was −0.23˚ (−0.627˚ to 1.56˚). There was no detectable increase in subsidence or rotation between 6 weeks and 5 years. We compared our data to that published for the Corail cementless stem and a similar pattern of migration was noted, however greater rotational stability was achieved with the Paragon stem over a comparable follow-up period.

The RSA results confirm that any minor motion of the Paragon cementless stem occurs in the first 6 weeks after which there is sustained stability for the next 5 years. The combination of a bi-planar wedge and transverse rectangular geometry provide excellent implant stability that is comparable to or better than other leading cementless stems.

Introduction

The marriage of rapid prototyping technologies with Arthroplasty has resulted in the fabrication and use of cutting jigs and guides which are tailored to a patients' individual anatomy. These disposable cutting blocks are designed based on input parameters obtained from pre-operative CT and MRI scans and manufactured using 3-D printers. Indirect benefits include a reduction in inventory and a decrease in the burden for central sterilising units. This approach is advantageous for the surgeon in the attainment of ideal mechanical alignment, which is known to be associated with an improved clinical outcome and implant longevity. This study evaluated the postoperative alignment parameters from a single surgeon series of patients following TKA with the Signature (Biomet) system.

In an effort to improve alignment in total knee arthroplasty (TKA), more recent prosthetic devices adapt computerised sculpting technologies based on preoperative MRIs to individualize surgical treatment. This is achieved by creating patient-specific surgical positioning guides for prosthetic alignment. Our study reports on the early clinical and functional outcomes and CT measured alignment of patients undergoing surgery with the Signature patient specific knee system.

We have reviewed the first one hundred patients selected to have a TKA using the patient specific knee system by a single surgeon over the last two years. Clinical and functional outcomes were assessed using the Western Ontario and McMaster Universities (WOMAC) index, the American Knee Society Scores (AKSS) and range of flexion at 6months. All data was analysed using a two tailed paired students t-test with statistical significance accepted at p<0.05. Post-operative CT scans were analysed to report on overall mechanical axial limb alignment, axial prosthetic tibial alignment, posterior tibial slope and femoral component rotation from the epicondylar axis.

Preoperative versus postoperative WOMAC scores for patients were 80.4 ± 2.2 and 45.2 ± 2.1 respectively. This was statistically significant at p=1.3×10–14. The AKSS pre- and postoperatively were 85.1 ± 4.6 and 151.9 ± 4.6 respectively with statistical significance reached at p = 1.3×10–13. Specifically, the pre- vs postoperative knee scores were 33.6 ± 2.8 and 75.1 ± 2.6 (p=3.9×10–12) while the function scores were 51.5 ± 2.8 and 75.8 ± 4 (p=3.4×10–7) respectively. Range of flexion preoperatively was 110.8 ± 2.8 while postoperatively was 122.1 ± 2.6 (p=0.0003). Postoperative CT scans revealed that the tibial axial alignment was 90.5 ± 7.7 degrees while the posterior tibial slope was 5.5 ± 0.3 degrees on average. In terms of femoral rotation, the epicondyllar axis was found to be 0.56 ± 0.1 degrees externally rotated with respect to Whiteside's line. The mechanical axis was 0.84 ± 0.1 on average. With all these measured parameters the number of outliers outside the accepted +/−3 degree range are small.

Our data demonstrates that the early results for knee replacements performed using the Signature patient specific jigs are very satisfactory delivering good clinical outcomes and an improved level of prosthetic alignment when compared to published data for standard instrumented knees.

The relationships between the transepicondylar axis (TEA), Whiteside's line(WL), and posterior condylar axis (PCA) are commonly used to determine the rotational alignment of the femur in total knee arthroplasty (TKA). It has been previously reported that may be gender differences in the rotational and mechanical anatomy of the distal femur1. The aim of our study was to examine the distal femur in a large number of patients to report on any gender differences within the group. The MRIs of a large cohort of prospectively chosen patients (n= 217) were examined retrospectively in order to determine the rotational femoral alignment. Varus/valgus relationship of their knees prior to prosthesis insertion was also examined. Measurements pertained to femoral rotation (relationships between WL, TEA and PCA) and varus/valgus alignment were calculated directly from MRI studies by a single observer. Gender differences were examined using an unpaired students t-test. Averages and standard deviations are reported to within two significant figures.

The posterior condylar axis was 2.6 ± 1.5 degrees relative to the transepicondylar axis and 91.8 ± 1.7 degrees relative to Whiteside's line. The varus to valgus ratio was 4.6 ± 5.9. Males in the group had a PCA of 2.4 ± 1.6 degrees relative to TEA compared to females in the group (2.8 ± 1.4 degrees). There was no significant difference between both groups (p=0.06). The PCA relative to WL was 92.1 ± 1.6 degrees for males compared to 91.6 ± 1.9 degrees for females with no significant difference between both groups (p=0.06). Finally, the varus to valgus ratio was 5 ± 5.7 for males compared to females (4.3 ± 6.2) with no statistical significance achieved between both groups (p=0.39).

Our results show that there is no significant difference in the rotational anatomy and varus/valgus alignment between men and women in a large cohort. Interestingly, the large standard deviation for values pertaining to femoral rotational anatomy (>3 degrees) suggest a significant degree of variability between patients. Thus, operative planning embracing our findings may prove to be of great clinical benefit by advocating individualising operative treatment in TKA surgery.