Total Hip Arthroplasty Performed with a Novel Design Type 1 Femoral Stem: A Retrospective Minimum 5-Year Follow-up Study

BACKGROUND: The Bencox M stem is a mid-short type 1 stem with additional unique design features. It has a reduced stem length and a lateral shoulder to facilitate minimally invasive surgery, as well as an angular lateral shoulder and a trapezoidal neck to minimize stem-liner impingement. There have been many mid-term reports on type 1 stems, but no results have been reported so far on this novel design type 1 femoral stem. This study presents the clinical and radiological outcomes of total hip arthroplasty performed with the M stem after a minimum 5-year follow-up. METHODS: From July 2014 to February 2015, 125 primary total hip arthroplasties using the M cementless femoral stem were performed on 112 patients in our hospital. Among them, 94 patients (106 hips) were eligible for the study and were followed up for more than 5 years. Our primary outcome was clinical results, which were evaluated by the Harris Hip Score (HHS), thigh pain, noise, and other complications. Secondary outcome was radiological outcomes. Seventy-seven hips were evaluated radiologically with attention to implant fixation, migration, loosening of component, degree of stress shielding, radiolucent lines, focal osteolysis, heterotopic ossification, and the evidence of impingement between the stem and liner. RESULTS: The average HHS improved from 54.6 points (range, 24–67 points) to 96.8 points (range, 91–100 points) at the latest follow-up. Three hips (2.8%) had intermittent thigh pain, which was tolerable without medication. Five hips (4.7%) had ceramic-related noise. There were no other complications such as infection, nerve injury, dislocation, or revision. All implants showed radiographic evidence of stable fixation by bone ingrowth without migration. Fifty-seven hips (74%) showed mild femoral stress shielding. Distal cortical hypertrophy was detected in 7 hips (9%), and heterotopic ossification was observed in 17 hips (22%). No implant demonstrated focal osteolysis and notching of the femoral neck or shoulder on radiographs. CONCLUSIONS: The minimum 5-year results of total hip arthroplasty performed with the M cementless femoral stem were encouraging clinically and radiologically. A long-term follow-up will be necessary to evaluate its longevity.