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A modified Gruen classification for comparing short and conventional Zweymüller-type stems: A three-dimensional
Takashi Yoneya1, Junichi Nakamura1, Shigeo Hagiwara1
1Department of Orthopedic Surgery, Graduate School of Medicine, Chiba University, 1-8-1 Inohana, Chuo-ku, Chiba City, Chiba, 260-0856, Japan.
Background:
The original Gruen classification is widely used for radiographic assessment of femoral stems in total hip arthroplasty (THA). However, when stems of different lengths are compared, the same zones may not correspond to equivalent anatomical regions, limiting interpretation of regional fixation patterns. This study proposed a complementary modified Gruen classification for comparing short and conventional Zweymüller-type stems and evaluated its utility using three-dimensional computed tomography (3D CT).
Methods:
We retrospectively reviewed 519 primary THAs performed through the direct anterior approach between 2013 and 2021. After propensity score matching, 91 hips with a MIRFY short stem and 91 with a Profemur Z conventional stem were included. Stem-cortex contact was assessed on postoperative 3D CT. In addition to the original Gruen classification, a modified classification based on reproducible femoral landmarks was used to divide the femur into proximal, middle, and distal regions. Contact was defined as the proportion of the stem surface in each region that contacted cortical bone. Radiographic bone reactions and thigh pain were also evaluated at 2 years postoperatively.
Results:
With the original Gruen classification, the short stem appeared to show greater contact in distal zones. In contrast, the modified classification reassigned these contacts to more anatomically proximal regions, suggesting that the original classification may distort regional interpretation when stems of different lengths are compared. Under the modified classification, the MIRFY stem showed a more proximal contact distribution than the conventional stem. Radiographic bone reactions at 2 years were similar between groups, whereas thigh pain was less frequent in the MIRFY group.
Conclusions:
The modified Gruen classification may serve as a complementary framework for comparing femoral stems of different lengths. This approach may allow more anatomically appropriate interpretation of regional stem-cortex contact when stems of different lengths are compared.
