Higher correction leads to more hinge fractures but does not affect short-term outcomes in medial opening wedge high tibial osteotomy in medial compartment osteoarthritis

INTRODUCTION

Unicompartmental medial osteoarthritis of the knee is often the result of overloading of the medial compartment due to a varus alignment of the lower limb. Correction of the alignment by doing a medial opening wedge high tibial osteotomy (MOW-HTO) offloads the medial compartment and also improves the functional outcomes in these patients.^[1,2]

The degree of correction often depends on the pre-operative malalignment of the knee, with the aim to make the mechanical axis or the Mikulicz line pass through near the middle of the tibial plateau, thus offloading the affected compartment.^[3,4] This study aims to find whether larger corrections lead to more intraoperative complications and whether they affect short-term outcomes as compared to smaller corrections.

MATERIALS AND METHODS

The study was designed as a retrospective cohort study in which 37 patients (46 knees) with medial compartment osteoarthritis were included. The patients were operated on between January 2020 and December 2023 in three different centers by three different surgeons. All surgeons had a similar surgical approach and followed the same rehabilitation protocol. All the osteotomies were fixed with standard anglestable locking implants.

All the patients included in the study were below 60 years of age and had isolated medial compartment osteoarthritis of the knee on radiological evaluation. Patients with fixed flexion deformity of the knee of more than 10°, or those with rheumatic arthritis of the knee, were excluded from the study. Patellofemoral osteoarthritis was not used as an exclusion criterion for the study.

Patients were clinically examined for isolated medial joint line tenderness, varus thrust gait, and pain relief on application of a medial offloader brace. Pre-operative radiographic evaluation included standard anteroposterior, lateral, and Rosenberg views of both knees, along with a long-leg standing radiograph of both lower limbs with the patellae facing forward with a radiological marker [Figure 1]. Magnetic resonance imaging was not routinely done in all cases. Pre-operative and post-operative functional scoring was done using the Oxford Knee Score (OKS).

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Figure 1:

(a) Pre-operative and (b) Post-operative long-leg alignment radiographs showing correction of limb alignment with the Mikulicz line passing through the center of the knee.

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Planning for the MOW-HTO was done using the standard Miniaci method, and the target post-operative Mikulicz line was aimed to pass through the point at 55% of the tibial plateau width measured from the medial border of the tibial plateau. Patients were then grouped into two groups based on the opening of the wedge. Patients requiring an opening of <10 mm were placed in one group, and those requiring an opening of more than 10 mm were placed in another.

All patients were placed supine on the operating table under spinal anesthesia, and a standard anteromedial approach to the medial tibia was taken. After dissection of the superficial layer and the pes anserinus bursa, the pes anserinus tendons were separated, and a window was created to access the medial collateral ligament (MCL). For patients in the first group, a two-window technique over and under the superficial MCL (sMCL) was used, sparing the structure. For patients in the second group, the sMCL was taken off the tibia using an L-shaped incision, sparing the proximal 1.5 cm of its incision. A blunt Cobbs-type periosteal elevator was then used to dissect the structure off the back of the tibia till the head of the fibula was reached. A radiolucent Hohmann retractor was used to protect the structures under the tibia, and then a standard ascending biplane type osteotomy was done with the ascending limb of the cut extending at an angle of 110° just beyond the tibial tuberosity [Figure 2]. A 2-mm Kirschner wire was used in an antegrade manner to protect the hinge while opening the osteotomy to the planned length with a lamina-spreader. A standard angle-stable locking plate was then used to fix the osteotomy. In case of a suspected hinge fracture, a hinge screw in the form of a 4-mm cannulated cancellous screw was used to replace the hinge wire [Figure 3]. The osteotomy site was left empty in both groups. In case the sMCL was taken off, it was repaired underneath the plate. A valgus stress test was performed in all cases, followed by closure of the subcutaneous tissue with absorbable suture and the skin. A suction drain was applied for 2 days in all cases. Post-operative radiographs were routinely analyzed for hinge fractures [Figure 4].

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Figure 2:

(a) Intra-operative fluoroscopy with the hinge protection wire in place, (b) Cutting along the guiding K-wires, (c) Intra-operative check of limb alignment.

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Figure 3:

(a) Application of hinge-wire in a suspected case of lateral hinge fracture, (b and c) Postoperative radiographs showing hinge-screws for lateral hinge fractures.

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Figure 4:

(a-e): Representative post-operative radiographs from group 1.

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Postoperatively, active range of motion and static and dynamic quadriceps and hamstring exercises were started from day 1. Patients were made to weight-bear as tolerated from 3 weeks postoperatively. In cases with a suspected or diagnosed hinge-fracture, the weight bearing was delayed to 6 weeks. Post-operative functional and radiological evaluations were done at 3, 6, and 12 months.

RESULTS

Of the 46 knees analyzed in the 37 patients, 23 knees required a wedge opening of <10 mm as calculated on the pre-operative measurement (mean 7.09 mm ± 1.38 mm). The mean pre-operative OKS in this group (Group 1) was 33.4 ± 2.8. The other 23 knees required a wedge correction of more than or equal to 10 mm (mean 11.91 mm ± 1.16 mm). The mean pre-operative OKS in this group (Group 2) was 29.9 ± 2.3.

Of the 23 cases in Group 1, there was one Takeuchi Type I fracture of the hinge on the lateral tibial cortex (4.3%) with the fracture line extending to the proximal tibiofibular joint. Of the 23 knees in Group 2, 10 knees (43.4%) had an intraoperative fracture of the hinge. Of these, two were Takeuchi Type II fractures with the fracture line propagating distally from the hinge point. The others were Takeuchi Type I fractures. The rate of hinge fracture was significantly higher (p = 0.0031) in the second group compared to the first.

In the early post-operative period of 3 months, the OKS improved in both groups. The mean OKS in Group 1 improved by a mean of 6.22 points, while that in Group 2 improved by 7.78 points. There was no significant difference (p = 0.095) in the improvement of OKS in both groups at 3 months.

In the interim and long-term follow-up of 6 and 12 months, the OKS continued to improve in both groups. The mean increases in OKS for Group 1 at 6 and 12 months were 8.78 and 9.87, respectively. For Group 2, they were 12.30 and 14.61 at 6 and 12 months, respectively [Figure 5]. The difference in improvement of OKS was significantly higher in Group 2 as compared to Group 1 at both 6 months (p = 0.0004) And at 12 months (p < 0.001), respectively.

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Figure 5:

Line chart showing relative improvement of Oxford Knee Score between the two groups at 3, 6, and 12 months interval.

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DISCUSSION

MOW-HTO is a joint-preserving surgery often done in cases of medial unicompartmental knee osteoarthritis. The surgery aims to offload the diseased medial compartment by shifting the weight-bearing mechanical axis of the lower limb from a pathological medial position to a relatively lateral one. Our study aims to find the correlation between the size of the opening wedge and the rate of complications and the functional outcome measures. The study shows that there is a significantly higher risk of hinge fracture as the size of the wedge goes above 10 mm. It also shows that there is no significant difference in functional outcome in the form of OKS at 3 months follow-up between the two groups. However, at 6 and 12 months follow-up, the group requiring larger correction shows significantly more improvement.

Lateral hinge fracture is a well-documented complication of MOW-HTO, with incidence rates varying between 3 and 30%^[5-7] across studies. Some studies report that using radiographs or intraoperative fluoroscopy to diagnose hinge fractures may cause underreporting of the same, and the rates could be as high as 50%^[8] if computed tomography is used to detect them.^[8,9] The size of the opening wedge has been identified as a predictive factor for hinge fracture, with studies proposing a cut-off ranging from 11 to 12 mm.^[10,11] Using a pragmatic cut-off of 10 mm has also been used to analyse the rate of hinge fractures, showing that <10 mm opening has a lower incidence^[12] as found in our study. For extraarticular hinge fractures, Takeuchi types I and II, a hinge screw is a valid treatment^[13,14] as was done in our study.

Early clinical outcome measures at 3 months in our study show no significant difference between the two groups. However, the second group requiring higher correction shows significantly more improvement at 6 and 12 months follow-up. The difference could be explained by the worse baseline score for the patients requiring the larger correction. Both groups showed improvement in the OKS throughout the duration of the follow-up, which is consistent with studies measuring the functional outcomes after MOW-HTO.^[1-3]The higher incidence of hinge fractures in the second group may explain the relatively poorer OKS at 3 months due to the protected weight bearing and slowed rehabilitation.

The clinical implication of this study is to alert to the higher risk of intraoperative complications in the form of hinge fracture associated with larger corrections in the MOW-HTO. However, the larger correction may provide a better functional outcome. However, the larger correction provides similar improvement in clinical outcome as compared to the smaller correction group. There is a tradeoff between the rate of intraoperative complications and post-operative functional outcome, and patients undergoing MOW-HTO should be counseled as such.

The limitations of the study would be the small sample size and the retrospective design of the study. We relied on intraoperative clinical picture and fluoroscopy as well as post-operative radiographs to diagnose the hinge fractures; a CT scan was not done in any case. This might lead to an under-reporting of the rate of hinge fractures, as previously shown in the cited literature. Furthermore, our study did not account for post-operative radiographic measures like the corrected mechanical axis and the angles to analyze whether they contributed to the increased rate of hinge fractures in the larger correction group. Although none of the knees showed any valgus instability, the differential handling of the sMCL may affect the early OKS, and this was not adjusted for in the study. We did not adjust our statistical analysis to account for the baseline difference in the OKS across the two groups, which could account for the significant difference in the outcome measures at 6 and 12 months. Longer follow-up and a larger sample size are required to come to a more significant conclusion.

CONCLUSION

MOW-HTO is an effective method to treat unicompartmental OA of the knee joint. Higher opening of the medial wedge increases the risk of mechanical complications in the form of a hinge fracture, but does not compromise the functional outcome of the surgery for a follow-up of 1 year. Patients should be counseled regarding these complications and benefits before undergoing the surgery based on the pre-operative calculations.