For decades, osteotomy planning has centred on bony geometry. Mechanical axis targets, correction angles and hinge position have been carefully calculated using weight-bearing radiographs.
But what if the apparent deformity is not purely osseous?
Soft tissue laxity, reflected in the joint line convergence angle, can distort preoperative measurements and contribute to unintended over-correction. A 2025 study by Hiranaka et al at the Sydney Orthopaedic Research Institute now provides compelling evidence that accounting for this variable significantly improves alignment accuracy in medial opening wedge high tibial osteotomy (MOWHTO).
The planning challenge: when bone-based calculations are not enough
Traditional osteotomy planning relies heavily on bony landmarks and long-leg weight-bearing radiographs to determine the target mechanical axis correction. Surgeons calculate the desired degree of valgus correction based on established principles, often aiming to shift the weight-bearing axis laterally to unload the medial compartment.
Yet, despite a surgeon’s best efforts, postoperative alignment does not always perfectly match the preoperative plan. One of the key contributors to this discrepancy is soft tissue behaviour, specifically medial and lateral ligamentous laxity.
The joint line convergence angle (JLCA) reflects the degree of convergence between the distal femoral and proximal tibial joint lines. In a varus knee with medial compartment disease, an increased JLCA often reflects medial laxity or asymmetrical soft tissue tension. Under weight-bearing conditions, this laxity can exaggerate apparent deformity.
If the correction is planned purely on the basis of bony alignment without accounting for this soft tissue component, there is a risk of over-correction once the osteotomy restores tension balance. Even a few degrees of unintended valgus can alter biomechanics in a way that may affect long-term joint health.
The study: accounting for the difference in JLCA
The 2025 study by Hiranaka and colleagues introduced and validated a practical method to address this issue.
Their differential JLCA or dJLCA method involves:
- Measuring the JLCA on a standing weight-bearing radiograph
- Measuring the JLCA again in a supine position, typically using CT
- Calculating the difference between the two values
This difference – the dJLCA – represents the soft tissue component of alignment that becomes apparent under load.
In the study cohort, a measurable difference between standing and supine JLCA values was identified. This differential acted as a valgising soft tissue factor that influenced the accuracy of correction if not accounted for during planning.
Incorporating the dJLCA adjustment reduced mean planning error in postoperative hip–knee–ankle alignment from 0.6° to 0.2°. While these numbers may appear small, in osteotomy surgery even sub-degree differences can be clinically meaningful.
A standing JLCA threshold of approximately 2.8° was also identified as a marker for cases at higher risk of planning inaccuracy.
A practical example of the dJLCA method
Consider a patient scheduled for medial opening wedge high tibial osteotomy for varus malalignment.
- Weight-bearing standing JLCA measures 4.3°
- Supine CT JLCA measures 1.6°
- The difference, or dJLCA, is 2.7°
If planning is based purely on the standing radiograph, the 4.3° measurement may contribute to calculation of a larger valgus correction. However, the 2.7° difference reflects soft tissue laxity that manifests under load rather than fixed bony deformity.
By subtracting this 2.7° valgising factor from the planned correction angle before surgery, the surgeon adjusts for the soft tissue contribution. This increases the likelihood that the final postoperative mechanical axis more closely reflects the intended target and reduces the risk of unintended over-correction.
In the study cohort, incorporating this adjustment significantly reduced alignment planning error, demonstrating that even small angular refinements can influence final limb position.
Implications for surgeons
For surgeons performing MOWHTO, this study reinforces several important principles:
- Alignment planning must consider both bone and soft tissue.
- Weight-bearing radiographs alone may overestimate the true bony deformity in knees with significant laxity.
- A simple preoperative comparison between standing and supine JLCA can meaningfully improve correction precision.
Importantly, the dJLCA method does not require complex new technology. It builds on imaging that is already commonly used in modern practice, particularly in centres utilising CT-based planning or patient-specific instrumentation.
Incorporating dJLCA data adds a further layer of precision for surgeons using computer-assisted planning, robotic platforms or advanced simulation tools such as Smart Osteotomy reporting. It allows correction strategies to move beyond fixed alignment formulas and toward patient-specific mechanical realignment.
Implications for allied health professionals
Physiotherapists and exercise physiologists play a central role in patient selection, prehabilitation and postoperative care.
Understanding the role of soft tissue laxity in osteotomy planning has several implications:
- Patients with apparent large varus deformities may have a significant soft tissue component contributing to that appearance.
- Restoration of alignment is not purely a bone procedure but also a rebalancing of ligamentous forces.
- Subtle differences in postoperative alignment may influence rehabilitation strategies, particularly in high-demand athletes.
Improved alignment precision may contribute to more predictable symptom relief and function during rehabilitation. It also reinforces the importance of comprehensive preoperative assessment and interprofessional communication.
What this means for patients
Modern knee realignment surgery is becoming increasingly personalised.
Not all varus knees are the same. Two patients may appear to have similar deformities on a standing X-ray, yet differ significantly in soft tissue behaviour. Accounting for these differences in the surgical plan enables surgery to be more closely tailored to each patient’s physiology.
Surgery will never be completely predictable but the dJLCA method represents a meaningful step towards more predictable correction and more personalised procedures.
Advancing precision in joint preservation
Medial opening wedge high tibial osteotomy remains a valuable joint-preserving intervention, particularly for younger and active individuals with medial compartment disease.
The dJLCA method contributes to the ongoing evolution of osteotomy planning by addressing a known but historically under-quantified variable – soft tissue laxity under load.
For surgeons, it provides a validated, practical strategy to reduce alignment error.
For allied health professionals, it deepens understanding of postoperative biomechanics.
For patients, it represents progress toward more individualised surgical care.
As knee preservation techniques continue to advance, refinements such as these underscore a central principle: precision in planning directly influences performance in practice.
Disclaimer
All information is general and not intended as a substitute for professional advice. Any surgical or invasive procedure carries risks.
References
- Hiranaka T, Davey C, Grasso S, Dal Fabbro G, Ahedi H, Fritsch B, Parker D. A validated method accounting for joint line convergence angle reduces planning errors in medial opening wedge high tibial osteotomy. Knee Surg Sports Traumatol Arthrosc. 2025 Sep;33(9):3341-3349. doi: 10.1002/ksa.12713. Epub 2025 Jul 13. PMID: 40652371. https://pubmed.ncbi.nlm.nih.gov/40652371/, [Accessed 23 February 2026]
- Park, JG., Kim, JM., Lee, BS. et al. Increased preoperative medial and lateral laxity is a predictor of overcorrection in open wedge high tibial osteotomy. Knee Surg Sports Traumatol Arthrosc 28, 3164–3172 (2020). https://doi.org/10.1007/s00167-019-05805-8, [Accessed 23 February 2026]
- Kumagai, K., Fujimaki, H., Yamada, S. et al. Difference in the early postoperative change of the joint line convergence angle between opening wedge and closed wedge high tibial osteotomies. J Orthop Surg Res 16, 66 (2021). https://doi.org/10.1186/s13018-021-02214-x, [Accessed 23 February 2026]
- Na, Y.G., Lee, B.K., Choi, J.U. et al. Change of joint-line convergence angle should be considered for accurate alignment correction in high tibial osteotomy. Knee Surg & Relat Res 33, 4 (2021). https://doi.org/10.1186/s43019-020-00076-x, [Accessed 23 February 2026]
