Abstract
Introduction: Using Automated Implant Design and Analysis (AIDATM ) a library of femoral CT scans was analyzed. Design goals of fit and offset allowed generation of nine sizes each of right and left implants for off-the-shelf use. The five to nine year follow-up of 96 (12 bilateral) consecutive primary total hip arthroplasties using this titanium alloy hydroxylapatite (HA) one-third coated femoral stem is reported. The purpose of this study is to determine if the incorporated engineering design goals produced results equal to or superior to other published mid-term follow-up studies.
Methods: Seventy-five femoral CT scans from patients undergoing cementless primary total hip arthroplasty were used as the anatomic database. The stem was conceived as a metaphyseal filling implant with flat posterior and lateral sides. Prior micro-motion cadaver research of other stems by the authors has shown that stems with the best fit on the Medial and Anterior quadrants as well as the Medial-Anterior corner (“MAMA” fit) are the most stable. Therefore the medial and anterior surfaces were contoured and were given priority fit. Distally the stem was cylindrical, and allowed for diaphyseal contact over a distance of two outer canal diameters. Five modular femoral heads were assumed with the design goal to be within 5 mm of the existing head center in 95% of cases.
Results: The median Harris score is 91% (max 100) with 77% good or excellent. The mean pain score is 39.5 (max. 44) with 80% having none or slight pain. Ten hips (11%) have slight to mild thigh pain. One stem was revised for late loosening due to osteolysis. All others remain well-fixed (Engh grade I). Six hips were re-operated: two head & liner changes for recurrent dislocations and four for wear and acetabular osteolysis. Radiographic analysis showed all stems became well bonded over the HA coating with no RLZ’s and accretion of bone in the lower half of Zones 1 & 7 at the end of the HA coating. There was no subsidence. Stress shielding was limited to the upper half of Zones 1 & 7. Osteolysis was common above the HA coating around the shoulders of the implant, but extended distally in only one stem (revised).
Discussion & Conclusion: These results at Five-year minimum follow-up compare favorably with other published series of cementless stems at the same follow-up. The 100% incidence of ingrowth combined with the absence of stress-shielding below the upper half of Zones 1 & 7, serves to validate the design concepts and methodology used to create this implant.
The abstracts were prepared by Nico Verdonschot. Correspondence should be addressed to him at Orthopaedic Research Laboratory, University Medical Centre, PO Box 9101, 6500 HB Nijmegen, The Netherlands.
