Medial Opening-Wedge High Tibial Osteotomy: To Fill or Not to Fill the Gap
Tomás Pineda, MD, CHILE Thomas Neri, MD, PhD, Prof., FRANCE Matthieu Ollivier, Prof, MD, PhD , FRANCE
ISAKOS eNewsletters
Current Perspective
2026
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Introduction
Medial opening-wedge high tibial osteotomy (MOWHTO) is a well-established technique for treating varus malalignment and medial compartment osteoarthritis by shifting load away from the degenerated compartment in order to delay or avoid joint replacement. Whether the resulting osteotomy gap should be filled with bone graft remains one of the most persistently debated technical questions related to the procedure and represents an area in which practice has been slower to change than the evidence would justify[1, 2].
Traditionally, bone-grafting was considered necessary to promote healing and prevent nonunion or loss of correction. Modern locking-plate fixation has since raised a genuine question about whether that assumption still holds, and a growing body of comparative evidence has started to answer it.
In our view, this evidence indicates that gap-filling is not strictly necessary in most cases but that it remains a reasonable and potentially beneficial option where available. In areas where resources are limited, the relevant question shifts from whether to fill the gap to identifying which patients are most likely to actually benefit from it. The present review synthesizes the highest-quality comparative and radiological studies on the topic rather than following a systematic review protocol.
Clinical Outcomes: Union and Function
Time to union follows a consistent pattern across comparative studies, autologous iliac graft has shown no consistent advantage in terms of time to union or maintenance of correction as compared with leaving the gap unfilled[3]. Pooled outcomes across comparative and non-randomized studies have shown no difference in knee function or loss of correction between grafted and non-grafted groups. Although autograft has been associated with a higher likelihood of complete radiographic union at one year, this association is based on low-certainty evidence[1]. Functional outcomes tell a more consistent story: knee scores have shown no reliable difference between grafted and non-grafted groups [1].
In well-stabilized osteotomies with an intact lateral hinge, a filler mainly speeds the pace of radiographic consolidation, not the eventual rate of union or the functional outcome.
Biological Rationale and the Role of Gap Size
The recent adoption of locking plates has changed the rules of the game. By minimizing micromotion at the osteotomy site, rigid fixation now carries much of the mechanical burden that grafting was once meant to address, particularly in small-to-moderate corrections.
Across a wide range of gap sizes, osteotomy healing has been shown to progress reliably without a graft, with mean gap filling reaching at least 90% by 2 years regardless of gap size, and no loss of correction, although larger gaps were associated with a slower rate of consolidation[4]. As the gap grows larger, that margin narrows: expert consensus supports the use of allograft for the treatment of larger opening gaps, whereas mechanical failure occurs disproportionately in knees with an unstable hinge fracture, regardless of gap size[2, 5].
None of this reduces cleanly to gap size alone. Correction magnitude, patient factors, and intraoperative hinge status interact to determine when filling actually helps (Figure 1). We see the lack of a fixed threshold as the more honest reading of the evidence, one that favors individualized judgment over a routine default.
Figure 1. Gradient of factors favoring an unfilled versus a filled osteotomy gap, based on correction magnitude, hinge stability, and patient risk factors.
Complications and Graft Selection
Mechanical failure appears to be more closely associated with gap size and hinge integrity than with filler choice. Larger opening gaps and unstable lateral hinge fractures have each been identified as independent risk factors for delayed bone healing and loss of correction, whereas a randomized trial comparing allograft and autograft directly found no difference in recurrence or complication rates between the two[3]. Smoking and unstable hinge fractures also delay consolidation independent of graft use or weight-bearing protocol.
Among fillers, comparative evidence has shown largely equivalent rates of union and recurrence, so the choice between them should be driven by harvest morbidity rather than by expected efficacy[3]. Autograft carries donor-site pain and hematoma at the harvest site. This equivalence has limits, however. Expert consensus has identified an increased risk of nonunion and infection with synthetic void fillers and points to allograft as particularly useful for larger opening gaps[2].
Practical Recommendations
Gap filling is not strictly necessary for most corrections. When the gap remains below roughly 10 to 12 mm and the hinge is intact, rigid locking-plate fixation alone achieves reliable union and function, and grafting adds cost and morbidity without a clear return. As the gap or correction angle grows, or when the hinge is compromised, the case for filling strengthens, since these conditions are associated with greater risks of mechanical failure and delayed healing (Figure 2).
Figure 2. Decision framework for filling the osteotomy gap.
When a filler is used, allograft is generally preferable to iliac autograft, given comparable outcomes without donor-site morbidity. When allograft is limited by cost or availability, we would reserve its use for patients with both a large correction and a compromised hinge or other healing risk factor, rather than grafting routinely.
Summary and Future Perspectives
Much of the current evidence comes from retrospective cohorts and small trials with short follow-up and inconsistent reporting of gap size and fixation method. Reintervention and hardware-removal rates, arguably the outcomes closest to a surgeon's cost-benefit calculus, are rarely powered as primary endpoints. Gap size offers a practical starting point for decision-making, but it is unlikely to capture the full picture when a surgeon has access to multiple filler options and applies them selectively rather than routinely. Bone quality, hinge stability, patient-specific healing risk, and material availability and cost can all shift the threshold at which filling becomes worthwhile; thus, two patients with a similar gap may reasonably be treated differently. Whether filling the gap also reduces early postoperative pain or local hematoma at the osteotomy site itself, distinct from donor-site morbidity, remains untested and warrants prospective evaluation. Future prospective, multicenter trials should account for these variables together, rather than stratifying by gap size alone, and should evaluate patient-reported outcomes and reintervention as primary endpoints. Such studies would help move the field from a fixed cutoff toward a more individualized approach to gap filling.
References
- Mao Y, You M, Zhang L, et al (2023) Lack of Efficacy of Bone Void Filling Materials in Medial Opening-Wedge High Tibial Osteotomy: A Systematic Review and Network Meta-analysis. Arthroscopy 39:1735–1757. https://doi.org/10.1016/j.arthro.2022.11.039
- Ollivier M, Claes S, Mabrouk A, et al (2024) Surgical strategy and complication management of osteotomy around the painful degenerative varus knee: ESSKA Formal Consensus Part II. Knee Surg Sports Traumatol Arthrosc 32:2194–2205. https://doi.org/10.1002/ksa.12273
- Haghpanah B, Kaseb MH, Espandar R, Mortazavi SMJ (2021) No difference in union and recurrence rate between iliac crest autograft versus allograft following medial opening wedge high tibial osteotomy: a randomized controlled trial. Knee Surg Sports Traumatol Arthrosc 29:3375–3381. https://doi.org/10.1007/s00167-020-06240-w
- Kim SJ, Nguyen LT, Seo Y-J, et al (2022) Bone healing on serial plain radiographs occurs slowly but adequately after medial opening wedge high tibial osteotomy without bone graft. Knee Surgery, Sports Traumatology, Arthroscopy 30:1795. https://doi.org/10.1007/s00167-021-06506-x
- Peez C, Milstrey A, Zderic I, et al (2025) The Type of Lateral Hinge Fracture in Medial Open-Wedge High Tibial Osteotomy Determines Its Stability: A Biomechanical Study. Am J Sports Med 53:1622–1628. https://doi.org/10.1177/03635465251332593
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