Abstract
Background:
For hip prostheses, short stems allow easy insertion and reduce thigh pain risk, and are therefore suitable for Minimally Invasive Surgery. However, clinical outcome depends on sufficient initial fixation in the proximal femoral component. Revelation stems are designed to increase medullary cavity occupancy in the proximal femoral component and allow physiological load transmission within this component. Theoretically, on initial fixation of the proximal part of the stem, fixation remains unaffected by cutting the distal part of the stem. Recently, the Revelation micro MAX stem has become available. In this system, only the distal part of the stem is removed. To prepare for the introduction of this stem, we evaluated its rotational stability by installing it in the femurs of formalin-fixed cadavers. We then evaluated the time course of changes in bone density at the stem circumference and stem position by CT in the first eight patients undergoing hip arthroplasty.
Subjects and Methods:
Micro MAX stems were inserted into the left femurs of one male and six female cadavers (76 to 95 years of age). A commonly used torque meter was mounted on the stem, and stem fixation was evaluated by the application of clockwise torque of 6 to 12 N-m. Further, in patients, three men and five women (age range 38–83 years, mean 67 years; two cases of femoral head necrosis, two of femoral neck fracture, and four of osteoarthritis of the hip) who underwent surgery with the micro MAX stem from July 2012 to April 2013 were evaluated at 3 weeks, and 3 and 6 months after surgery for stem insertion angle and stem subsidence by CT, and for bone density around the stem by the DEXE method.
Results:
Rotational stability of the micro MAX stem in cadaveric femurs was similar to that of the conventionally used Revelation stem. In CT examination after surgery, the micro MAX stem tended to be inserted in a slightly varus position. No evidence of stem subsidence was observed in eight patients, but progression of the varus was seen in one. Bone density in the stem circumference was maintained.
Discussion:
Although short stems have a number of advantages, problems with the first fixation might result in loosening. Allowing for the small number of patients and limited range of clinical conditions, our findings suggest that the rotational stability of the micro MAX stem is similar to that of the conventional stem. Postoperative CT measurement indicated that the micro MAX stem tended to be inserted in the varus position, particularly in patients with a large medullary cavity. The micro MAX stem was stable, and no decrease in bone density was seen. As with other short stems, however, care is required to avoid insertion in the varus position in patients with a large medullary cavity.
Conclusion:
This investigation identified no problems in initial fixation with the micro MAX stem. Clinical outcomes with this system should be favorable.
