Abstract
Introduction
Modularity in femoral stem designs allow surgeons to independently control leg length, offset, and femoral version in revision or complex primary THA cases. Initial enthusiasm in these modular stems has been tempered by recognition of modular junction failures. This study evaluates mean 5-year clinical results and survival rates of a 3-part titanium alloy modular femoral implant with unique taper geometries and a metaphyseal plasma spray surface. The current results are presented after pre-market independent fatigue testing performed by Orthopaedic Laboratory (Greenwald) and previously published early clinical results in 2006. Low plasticity burnishing (LPB) was added in 2005 to further strengthen the neck metaphyseal modular junction. The modular stem component is a polished cylindrical splined clothespin design. Our hypothesis is that these unique modular junctions succeed in offering the advantages of modularity without failure at this midterm follow-up period.
Methods
Between May 2010 and July 2016, 32 total hip arthroplasties were performed using a 3-part femoral stem with neck-metaphyseal-stem modular junctions. Surgeries were either the final stage of a two-stage revision for infection, revision THR for loosening, or a revision of a previous non-prosthetic replacement procedure. Patients were entered into an IRB-approved registry and followed with x-rays, HHS, Oxford scores, and patient satisfaction scores. Patients who failed to return for routine follow-up were contacted by phone or email. Two patients had died with their implants intact. Six patients could not be reached for an updated follow-up. One stem was revised for loosening at 33 months due to failed osseointegration in a patient with chronic renal failure. This removed stem was submitted for taper exam and sectioning.
Results
There were 23 patients for evaluation at a mean 61 months (range 21–98). Mean patient age at implantation was 56 (range 25–88), BMI was 27 (range 20–40). There were no modular junction failures. Modular junctions examined in the retrieved implant did not demonstrate any abnormalities other than normal wear properties. HHS and OHS scores both improved between pre-op and final follow-up, 23 to 85 and 17 to 43, respectively. Average patient satisfaction score at final follow-up was 9.8 out of 10 (min 8, max 10). Radiographic examination showed stem subsidence > 2mm and radiolucencies around the metaphyseal cone in 1 patient, the same patient who required implant removal. Radiolucencies were seen along the polished stem tip in 43% of cases, spot welding at the distal metaphyseal cone in 67% of cases, and mild proximal-medial stress shielding in 33% of cases.
Conclusion
This unique 3-part modular stem with metaphyseal fixation shows good functional and radiographic results at 5-year follow-up. There were no junctional failures. One stem was removed due to failure to osseointegrate and showed no worrisome taper abnormalities. Spot welding is common around the plasma spray metaphyseal cone. Implant removal, if necessary, only requires disruption of the metaphyseal fixation. Further follow-up will be important to confirm our confidence in this unique stem design.
