# Tibial morphological variations reveal medial proximal tibial angle discrepancies in Japanese patients: Three‐type classification for high tibial osteotomy

## TL;DR

This study identifies three types of tibial bone shapes in Japanese patients with knee osteoarthritis, showing that assuming equal angles during surgery can lead to errors.

## Contribution

A new three-type classification system for tibial morphology is introduced to improve surgical accuracy in high tibial osteotomy.

## Key findings

- Three clinically relevant tibial morphological types were identified: medial bowing, straight, and lateral bowing.
- Non-straight types showed significant discrepancies between mechanical and anatomical angles, affecting surgical planning.
- 42.5% of patients may experience intraoperative measurement errors if morphological types are not considered.

## Abstract

This study aimed to characterize the tibial morphology among Japanese patients with varus knee osteoarthritis and examine whether the mechanical and proximal anatomical medial proximal tibial angles (mMPTA and paMPTA) are equivalent in high tibial osteotomy (HTO) planning.

In this retrospective study, 193 varus‐aligned knees (hip–knee–ankle [HKA] angle < 0°) from 209 Japanese patients undergoing total knee arthroplasty or HTO were analyzed. Full‐length lower limb radiographs were used to assess tibial morphology, with the tibia divided into proximal, middle, and distal segments. Based on the proximal–middle and middle–distal angular relationships, nine morphological subtypes were identified and subsequently grouped into three clinically relevant categories: medial bowing, straight, and lateral bowing. The discrepancy between mMPTA and paMPTA (ΔMPTA) was calculated for each morphological category. Data were analyzed using Kruskal–Wallis test, chi‐square test, paired t‐test, and ANOVA.

Tibial deformities were highly prevalent, with proximal and distal deformities observed in 67.9% and 50.8% of knees, respectively. Among the 193 knees, 49 (25.4%), 111 (57.5%) and 33 (17.1%) were classified as medial bowing, straight type, and lateral bowing types, respectively. Significant differences in ΔMPTA were observed among the three groups (p < 0.001). Medial bowing type showed a significant negative ΔMPTA ( − 1.5° ± 0.5°, p < 0.001), straight type showed negligible discrepancy (0.0° ± 0.7°, p = 0.60), and lateral bowing type demonstrated a significant positive ΔMPTA ( + 1.3° ± 1.0°, p < 0.001).

Japanese patients with varus knee osteoarthritis exhibit complex tibial morphologies involving both proximal and distal segments. These morphological variations significantly influence the relationship between mMPTA and paMPTA. In 42.5% cases (non‐straight types), conventional equivalence assumptions may lead to intraoperative measurement errors during HTO verification. Preoperative morphological type identification and corresponding intraoperative adjustments based on predictable ΔMPTA values may enhance surgical accuracy and clinical outcomes.

Level III.