Comparative Study for Postoperative Initial Fixation Patterns of Two Different Types of Cementless Short Stem Using Three-Dimensional Templating Software

Background: While cementless short stems have become popular in total hip arthroplasty (THA), Metha is a relatively recent development that differs from other short stems in its initial fixation concept of partial collum-sparing metaphyseal anchorage. The purpose of this study was to quantify the contact state between Metha and the femur. Additionally, we investigated the difference in contact points between Meta and Fitmore, which is one of the more popular curved short stems. Methods: We conducted a retrospective review of 42 hips that underwent THA using Metha and 41 hips using Fitmore. Stem to-femur contact was evaluated by density mapping using a three-dimensional digital template system to quantify the contact condition according to the modified Gruen zone. The criterion for the stem-to-bone contact boundary was defined as a computed tomography value of 543 Hounsfield. Results: Quantitative evaluation of Metha according to the modified Gruen zones showed the ratio of surface area with high cortical contact in each zone. The results were 4.6% +/- 5.7% in zone 1, 0.9% +/- 2.3% in zone 2, 19.1% +/- 12.9% in zone 3, 1.4% +/- 3.2% in zone 5, 29.6% +/- 16.4% in zone 6, and 25.1% +/- 17.7% in zones 7. Evaluation of Fitmore for the same zones was as follows: 1.6% +/- 2.4%, 18.5% +/- 16.9%, 20.8% +/- 17.4%, 12.7% +/- 12.8%, 3.7% +/- 5.8%, and 13.3% +/- 10.3%. Comparing the two groups, the contact area was significantly greater for Metha in zones 1, 6 and 7 and Fitmore in zones 2 and 5 (p < 0.05). Conclusions: It is possible for Metha to achieve metaphyseal anchoring by contacting the cortical bone at the proximal femur, thus avoiding proximal offloading. To the best of our knowledge, no previous studies have quantitatively reported stem-to-cortical bone contact conditions in curved short stems.