MSA™ Stem design objectives
The design approach was undertaken to improve proximal load transfer while creating a bone and tissue sparing stem. The stem should be simple in design, amenable to reproducible technique and provide for fine tuning of joint mechanics while stimulating and maintaining compressive loads at the medial calcar.
In theory neck retaining devices provide for:
• Bone and/or tissue conservation
• Restoration of joint mechanics
• Minimal blood loss
• Potential reduction in rehabilitation
• Easily convertible to traditional THA in case of revision
• Simple reproducible surgical technique
• Opportunity to have bearing femoral head diameter and material as best indicated
• The selection of any standard surgical approach to the hip
• The prosthesis is compatible with large head technology
The MSA™ Stem, Total Hip System System selectively addresses new clinical trends in THA.
Design Rationale
MSA™ Stem, Total Hip System
Total Hip Arthroplasty (THA) has been and is an excellent surgical treatment for diseases of the hip joint. Cementless designs appear to be growing in market acceptance and demonstrate good to excellent clinical results. In spite of these encouraging results there is a growing demand for more conservative surgical approaches to hip disease and surgical intervention. Undesirable bony structural changes can occur after stemmed THA. We can improve bone remodelling and surgical approach in terms of tissue preservation.
Reviewing historical hip designs, Michael Freeman advocated neck-sparing features in early hip designs from the 1970’s. He demonstrated, as did Leo Whiteside, that there is a mechanical advantage in retaining the femoral neck. Both torsional and axial loads are reduced along with bending moments on the femoral component. Freeman, Whiteside and Charles Townley all advocated neck retention with conventional length total hip stems.
Professor Pipino from Milan, Italy has been promoting the use of neck retention short curved stems for the past 25 years. He has taken the conservative approach even further by advocating bony and soft tissue sparing techniques for THA. Some of the historical problems of the past address architectural changes that occur in the proximal femur after stem implantation. The challenge has been to create a design that loads the medial calcar in compression, maintaining the integrity of that bony structure. Some short stems require significant bone removal from the lateral femur, risking fracture of the greater trochanter. These designs also face considerable difficulty should removal be necessary.
Improved metal bearings have rekindled the interest in metal on metal Hip Resurfacing (HR). This approach is not tissue sparing and has demonstrated a significantly longer learning curve than traditional THA (primarily femoral preparation) with results at eight years closer to 95% survivorship1. At first glance, one would think these results are comparable, however we must remember that indications for HR are at most 10–15% of overall THA.
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