CLINICAL AND RADIOLOGICAL OUTCOMES OF SHORT STEM THA IN MINI INVASIVE NAVIGATED THA: ‘A POST OPERATIVE CT- SCAN STUDY OF METHA IMPLANT’

Abstract

Proximal load fixation in THA is paramount for maximum bone preservation and thus longevity of the implant. Conventional femoral stems may not achieve satisfactory proximal fixation in proximal/ distal femoral canal mismatch as in champagne type of femur, mal-united fractures, excessively bowed femur and some young large patients. Such mismatch can lead to uneven loading and a higher incidence of loosening or periprosthetic fracture. This risk is becoming higher with the recent trend to mini-invasive surgery. As an alternative to standard stem, the short stem femoral prosthesis was originally developed in the 1950’s and has recently been making a resurgence as a design conducive to the minimal invasive techniques, and allows for greater preservation of the natural anatomy. Our objective is to measure the clinical and radiological outcomes of the Metha^® hip short stem (a cementless, metaphyseal fix conserving diaphysis implant) with computer assisted technique and minimal invasive surgical approach.

This study is a single center, open label, data collection study of 35 consecutive subjects undergoing OrthoPilot^® navigation, Metha^® stem plasmapore cup system with minimal invasive muscle sparing surgical approach. (B. Braun Aesculap; Tuttlingen, Germany). Pre-Operatively an IRB Informed Consent, no exclusion specific to the implant, Harris Hip Evaluation and routine hip/pelvis plain xray was performed. Post-Operatively, a CT Scan within 6 months, repeat Harris Hip Score (HHS) Evaluation. The implant is made of titanium alloy with plasmapore coating in @ 2/3 of the implant (< 8 cm engages in proximal femur with a total length average of 10.75 cm)(range 9.75–11.75). The bearing surface is metal/ highly cross linked polyethylene. Size 32 head of modular neck with plasmapore shell. All cases were done thru muscle sparing anterolateral mini invasive approach.

At 6 months ± 1 month with an average age = 67, BMI= 37 (one was 360 pounds), Harris score was 46 preop and 96 post op.No thigh pain, No dislocation, Trendelenberg negative. Patient satisfaction was high. One fell had a non displaced fx of the neck that did not require revision but slowed full weight bearing for 6 weeks. One subsided in a 360 pound patient after he went full wt bearing while getting out of the shower a week after surgery. This was revised with a standard stem. The tip of two implants was discovered to have penetrated the proximal femur with no symptoms at 3 months period and no restriction to weight bearing during that period. This happened in the first 5 cases in which we modified technique with no other penetrations. Xray/CT scan showed no subsidence good bony ingrowth no radiolucency or loosening. The two penetrated corteses showed intense bony overgrowth on the two protruded tips of stem.

We conclude that cementless metaphyseal short stem with proximal plasmapore coating is associated with excellent clinical and radiological results. The technical errors we faced in the early two cases were identified and avoided by using awl canal finder with suction tube palpation of canal and direct visualization of the metaphyses before hammering the broach. The combination of the short stem prosthesis, modularity of the neck, computer assisted technique and minimal invasive arthroplasty is expected to reduce recovery time, increase the potential for successful restoration of function and add to the longevity of the prosthetic joint.