1 Introduction

Lateral hinge fracture (LHF) in medial opening wedge high tibial osteotomy (MOWHTO) occurs when the lateral cortical hinge, which serves as the pivot point for opening the osteotomy, fractures unintentionally. It is one of the most common complications of medial opening wedge high tibial osteotomy (MOWHTO).^1–3 Various studies have reported the incidence of lateral hinge fractures (LHF) ranging from 3 % to 34 %, primarily based on intraoperative and postoperative radiographic and CT scan findings. Several risk factors have been identified, including the location of the osteotomy, hinge position, and osteotomy width. However, medial opening gap width (ranging from 11 mm to 12 mm) remains the most consistent predictor.^2,4–11 A high endpoint, oblique hinge line, age, body mass index (BMI), hip-knee-ankle angle, fibular position, osteotomy location, and wedge-hinge relation have been analyzed for their correlation with LHF.^3,11–16 Patients with lateral hinge fractures (LHF) have considerably longer union time and significantly higher rates of non-union.¹⁶ LHF can lead to osteotomy displacement and recurrent varus malalignment before union in up to 12 % of cases.¹⁷ If the disruption of the lateral cortex is not recognized or addressed during surgery, it can cause instability at the osteotomy site, resulting in loss of angular correction, delayed union, non-union, or even implant failure. Lateral cortex fractures reduce axial and rotational stiffness and increase micromotion at the osteotomy site, further contributing to delayed union, non-union, and loss of correction.^18–21 Lateral hinge fractures (LHF) are classified based on the time of presentation as either acute or delayed. Acute fractures occur during surgery, while delayed fractures are those identified after one month postoperatively, typically during rehabilitation and follow-up radiographs.⁴ The reported incidence of delayed fractures account for 23 %–60 % of total LHFs in medial opening wedge osteotomy, highlighting that a significant number present later during follow-up. Studies on CT scan have found that most lateral hinge fractures (LHFs) occur intraoperatively, CT scans detect up to 50 % of cases, compared to 14.6 % on radiographs, with nearly 50 % are diagnosed later.^4,5,13,14 Majority of acute LHFs go undetected in intraoperative fluoroscopy and immediate postoperative radiographs, often being misclassified as delayed fractures. Misdiagnosis can worsen outcomes and increase complications after osteotomy, making timely detection of acute LHFs crucial. Additionally, risk factors and predictors of delayed LHFs remain largely unexplored. KC Kim et al.²² recommended MRI for suspected occult hip fractures, emphasizing their role in improving fracture detection rates. Magnetic Resonance Imaging (MRI) is crucial for early LHF detection, identifying subtle stress lines and edema. Its high sensitivity reveals bone marrow edema and periosteal reactions before fractures appear on radiographs.²³ This enables the diagnosis of acute occult or at-risk LHFs, allowing timely intervention to prevent progression.

While CT is well-studied and essential for diagnosing LHF, MRI may offer comparable or superior detection without radiation exposure. Lubovsky et al.²⁴ found MRI more accurate than CT for early occult hip fracture diagnosis, emphasizing its advantage in immediate imaging. However, no study has yet evaluated MRI's role in detecting LHFs in MOWHTO. This study therefore aims to: (1) Assess LHF frequency after MOWHTO using postoperative MRI alongside radiographs, (2) Identify missed acute LHFs and differentiate them from true delayed LHFs, and (3) Evaluate associated complications. We hypothesize that LHFs are more frequent than detected on radiographs and CT scans and that many LHFs most commonly occur intra-operatively but are often missed on post-operative radiographs and misclassified as delayed fractures.

2 Materials and methods

This study is a retrospective, nonrandomized study reviewed 250 knees with varus deformities from 227 patients (169 females [74.6 %], 58 males [25.4 %]) treated with MOWHTO at our hospital. All surgeries, performed by a single surgeon, occurred between January 2014–June 2017 and January 2023–January 2024. The affected side was nearly evenly distributed, with 131 cases (52 %) on the right and 119 (48 %) on the left. The average patient age was 60.4 ± 13.2 years (range: 34–83). Patients underwent clinical and radiographic evaluations at 4 weeks, 6 weeks, 2 months, 3 months, 4 months, 6 months, 1 year, and annually thereafter, with a minimum follow-up of 1 year.

Inclusion criteria: Active patients with symptomatic medial osteoarthritis or articular cartilage lesions of the knee joint, varus limb malalignment, and an intact lateral joint compartment or cartilage lesions with an International Cartilage Repair Society (ICRS) grade of less than I.

Exclusion criteria: Active knee infection, severe patellofemoral osteoarthritis, lateral femorotibial angle of 190° (10° anatomic varus alignment) or more, flexion contracture over 15°, and varus/valgus instability greater than 10° on a stress view with a Telos device (Telos, Marburg, Germany).

The study was approved by our institutional review boards, and informed consent was obtained. Medial opening-wedge HTOs were performed using the TomoFix™ plate (Synthes, Oberdorf, Switzerland).

2.4

2.4 Radiologic evaluation

Intra-operative fluoroscopy and post-operative radiography were performed with four views: antero-posterior, antero-posterior tibial slope (10° cephalad tilt), 15° internal rotation and lateral view. On post-operative day 2, a 1.5T multi-planar MRI was obtained. Radiographs and CT scans were assessed during follow-up visits. Fractures were diagnosed in CT scans by hypodense fracture lines in the cortex. The antero-posterior tibial slope view minimized parallax at the osteotomy site, ensuring full visibility of the osteotomy gap.²⁸ Loss of correction was measured by the Hip-Knee-Ankle (HKA) angle on standing full-length radiographs at day 4 post-surgery and during the 4-week follow-up.²⁹ Acute LHFs were defined as those detected immediately post-operatively via fluoroscopy, radiographs, or MRI. Missed acute LHFs were first identified on MRI but not visible on radiographs, while delayed LHFs appeared later in follow-up radiographs or CT scans but were not visible on immediate MRI or radiographs. We assessed MRI in the sagittal and coronal views of the lateral zone of the tibia. The lateral zone is a bony bridge located laterally to Hinge line (an antero-posterior line tangent to the medial edge of the fibular head) Fig. 1 on the axial plane of the CT scan.¹² A fracture was defined on MRI if at least one cut section of the lateral zone showed a linear low-signal intensity (SI) was surrounded by an intermediate-SI area on T1WI and the linear low SI was surrounded by high SI on T2WI, extending from the endpoint of the osteotomy to the lateral cortex Fig. 2. Bone union was considered radiologically complete when bridging callus reached Zone 3, with at least 50 % of the osteotomy gap bridged with callus.²⁷ After collecting data with a minimum follow-up of one year, we categorized the cases into four study groups- Group 1: Acute fractures detected on radiographs and MRI, Group 2: Acute fractures detected only on MRI (missed lateral-hinge fractures), Group 3: True delayed fractures, and Group 4: No fracture detected under any modality Table 1.

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Fig. 1 Hinge line, Lateral zone and LHF visualized on Coronal cut sections at different sections of Lateral zone (A,B,C). Description: The lateral zone is a bony bridge located laterally to Hinge line (an antero-posterior line tangent to the medial edge of the fibular head). (A) coronal cut at the anterior portion of Lateral zone, (B) coronal cut at the middle portion of Lateral zone, (C) coronal cut at the posterior portion of Lateral zone.

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Fig. 2 Sagittal cut of lateral portion visualizing no fracture (A,B) and fracture (C,D), Coronal cut of lateral portion visualizing no fracture (E,F) and fracture (G,H).

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Table 1 Categorization of the cases into four study groups.

	Fracture on immediate radiographs	Fracture on immediate MRI	Fracture on Follow-up radiographs/CT:	Total
Group 1 (Acute LHF)	Yes	Yes	Yes	149
Group 2 (Missed LHF)	No	Yes	Yes	70
Group 3 (True delayed LHF)	No	No	Yes	4
Group 4 (No fracture)	No	No	No	97

3 Results

This study includes a total of 250 knees among 227 patients. Of them, 169 (74.6 %) are females and 58 (25.4 %) are male. The affected side was nearly evenly distributed, with 131 cases (52 %) on the right side and 119 cases (48 %) on the left side. The average patient age was 60.4 ± 13.2 years (range: 34–83 years). The mean BMI was 24.46 ± 5.63 kg/m² (range: 2.4–35.7 kg/m²). Bone Mineral Density (BMD) values averaged −1.45 ± 1.73 (range: 4.0 to 2.9).The average opening distance was 7.93 ± 2.98 mm (range: 4.0–20.0 mm).

A total of 250 knees were analyzed: 149 had fractures visible on MRI, and 101 showed no fractures. Additionally, 83 knees had fractures visible on radiographs or CT, with 18 diagnosed intraoperatively or immediately post-operatively, and 65 identified later on follow-up radiographs or CT. This indicates that radiographs/CT missed 44.3 % of lateral hinge fractures. Notably, 4 fractures were initially undetected on radiographs or MRI but appeared on follow-up imaging. Chi-square test showed no statistically significant difference in gender distribution across the groups. ROC analysis of age distribution revealed an Area Under the Curve (AUC) of 1.0, indicating perfect predictive ability, with an optimal age cut-off of 61 years for a high risk of lateral hinge fracture. Pairwise comparisons revealed significant BMI differences between Group 3 compared to Group 4. ROC analysis also showed an AUC of 1.0, with an optimal BMI cut-off of 30.06 for a high risk of Delayed lateral hinge fracture. A BMD cut-off of −2.8 was identified, beyond which differences became significant. Additionally, a critical cut-off of 7.5 mm was determined, beyond which MRI-detectable fractures increased significantly. Analysis of time to Zone 3 bone union showed that Group 1 averaged 5.81 months, Group 2 averaged 6.65 months, Group 4 had the shortest time at 4.58 months, and Group 3 had the longest at 8.25 months. The chi-square test revealed no significant differences in infection or complication rates among the groups (Table 2).

4 Discussion

This is a novel study on the role of MRI in Lateral hinge fractures in MOWHTO. The most important finding of the study is that 149 of the 250 cases (59.60 %) showed visible fractures on MRI, compared to only 33.2 % that were visible on radiographs and CT scans. This is significantly higher than what is typically detected using radiographs and CT scans, indicating that the frequency of lateral cortical hinge fracture (LHF) complications is grossly underestimated after MOWHTO. If we were to exclusively rely on radiographs and CT scans, we would have diagnosed only 83 cases of fractures compared to 149 cases identified through MRI. Of these 83 cases, a very small fraction, 18 out of 83 (21.68 %), were diagnosed during the intra-operative or immediate postoperative period using fluoroscopy and radiographs exclusively, while the remaining 65 out of 83 (78.31 %) were missed. Additionally, only 4 out of 83 (4.81 %) of the total fractures were confirmed as true delayed lateral hinge fractures. This means incidence of delayed fracture is 1.6 % (4 out of 250) in total cases of medial opening wedge HTO. The remaining 61 out of 83 (73.49 %) fractures, initially thought to be delayed, were actually fractures that had been missed on radiographs. This data highlights MRI's critical role in identifying missed acute lateral hinge fractures and distinguishing them from true delayed fractures. It also suggests that complications like delayed union or non-union are often linked to fractures missed on radiographs and CT scans. Suh et al.³⁰ found a 25 % incidence of hinge fractures on MRI, similar to CT detection rates, indicating MRI may offer superior detection without radiation exposure. Studies by Lee BS et al.⁴ and Lee OS et al.⁵ showed lower LHF detection rates with CT (31.4 % and 24.5 %, respectively), with nearly 50 % of fractures missed on radiographs. Kim JH et al.¹⁴ and Nha KW et al.³¹ also reported lower CT detection rates than MRI. These findings suggest CT scans may miss a significant number of fractures compared to MRI. Martin R et al.⁸ reported an 8 % incidence of delayed hinge fractures (26 out of 323 cases), comprising 40.62 % of total fractures. Lee BS et al.⁴ found an 8.2 % incidence (14 out of 170 cases), accounting for 25 % of lateral hinge fractures. Other studies by Dexel J et al.³² and Jung WH et al.²⁷ reported delayed fractures between 8 % and 14 % (Table 3). In contrast, our study found a lower incidence of 1.6 %, suggesting the actual rate of delayed fractures may be lower than previously reported, potentially due to missed acute fractures that later appear on radiographs and CT scans, misdiagnosed as delayed fractures. Seo SS et al.¹⁰ and Lee SJ et al.¹¹ identified a medial opening distance >11 mm as a risk factor for lateral cortex fractures. In contrast, our study found a lower critical cutoff of 7.5 mm, below which MRI-detectable fractures significantly increased, suggesting a smaller threshold than radiograph and CT-based studies. Gender distribution was not a significant risk predictor. Age was significant only between Groups 1 and 2, with a high-risk cut-off at 61 years. BMI strongly correlated between Group 3 and 4, with a risk threshold >30.06, suggesting BMI >30.06 may increase the risk of delayed lateral hinge fractures. Fracture groups had significantly lower BMD than non-fracture groups, with a high-risk cut-off at BMD ≤ −2.8. This suggests a link between lateral hinge fractures, older age, higher BMI, and lower BMD, with a strong positive correlation between BMI and BMD. Group 3 (delayed fractures) had the highest mean age (66.5 years, SD: 10.61), and were more common in older patients with higher BMI and lower BMD, indicating metabolic or biomechanical factors may influence delayed development of fractures. Group 4 had the shortest fracture healing time, indicating faster healing in cases with no fractures. Cases with fractures had longer healing times with true delayed fractures (Group 3) taking longest to heal, emphasizing the impact of delayed fractures. ANOVA results for loss of correction showed significant differences between fracture groups compared to No-fracture group, highlighting their impact on osteotomy outcomes. The loss of correction (p = 0.440) showed no significant difference between Groups 1 and 2, suggesting fracture detection modality does not affect post-operative alignment. Infection and other complication rates did not significantly differ between groups, regardless of fracture detection modality. MRI's necessity for diagnosing lateral hinge fractures (LHFs) in MOWHTO is debated due to its cost, availability, patient contraindications, and procedure time. However, MRI is crucial for early detection of LHFs, identifying subtle stress lines and edema, and visualizing bone marrow edema and periosteal reactions before fractures appear on radiographs. This study suggests CT scans may miss many fractures, particularly delayed ones, as their detection rate is lower than MRI. MRI offers more detailed imaging, improving diagnostic accuracy and avoiding the radiation exposure of CT, making it a safer option for repeated imaging (Fig. 3).

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Fig. 3 Intra-operative fluoroscopy (A), immediate post-operative Radiograph (B) and CT scan (C) and MRI (D, E), follow-up radiograph and CT scan images at 4 weeks (F), 8 weeks (G), 3 months (H) and 5 months (I). Description: Case example demonstrating a missed acute LHF: Intra-operative fluoroscopy, immediate post-operative radiograph, and CT scan showed no fracture; however, the fracture was evident on the post-operative MRI. On follow-up at 4 weeks, the fracture was visible on CT scan.

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5 Limitations

This study has several limitations: the comparison of MRI and CT detection rates relied on previous CT-based studies. All cases underwent biplanar osteotomy, which may affect fracture incidence and complications compared to uniplanar osteotomy. Recording bias may have occurred, as a single surgeon made diagnoses, preventing inter-observer reliability evaluation.

6 Conclusion

In conclusion, our results suggest that lateral cortical hinge fractures after MOWHTO are often underestimated, with true delayed fractures being relatively rare. Most LHFs occur intra-operatively but are frequently missed on radiographs and CT scans; these fractures become apparent later in follow-up and are misdiagnosed as delayed fractures. Immediate post-operative MRI improves diagnostic accuracy, distinguishing early LHFs from delayed fractures. True delayed LHFs have higher rates of delayed union. Despite cost concerns, MRI's high sensitivity is essential for preventing postoperative complications. Early detection through MRI allows for timely modifications in rehabilitation protocols, reducing risks such as loss of correction or nonunion, ultimately optimizing outcomes.

CRediT authorship contribution statement

Woon-Hwa Jung: Conceptualization, Writing – review & editing. Minish Raghunath Katkar: Formal analysis, Investigation, Writing – original draft. Min-Seok Seo: Data curation, Methodology. Dong-Hyun Kim: Project administration, Resources. Ryohei Takeuchi: Supervision, Validation.

Patient consent

Informed consent was obtained from all individual participants included in the study.

Ethical approval

All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.

Funding source

The author(s) declare no external funding source and no financial support for the conduct of the research and/or preparation of the article.