Medial opening wedge high tibial osteotomy (MOWHTO) is a widely recognized and effective surgical procedure for correcting varus knee deformity, a condition often associated with medial compartment knee osteoarthritis. The procedure involves making a precise cut in the tibia to create a wedge-shaped opening, which repositions the weight-bearing axis of the leg to the healthier, lateral side of the knee. This can help alleviate pain, slow down disease progression, and delay or even prevent the need for a total knee replacement. However, a significant intraoperative complication is the lateral hinge fracture, which can compromise the stability of the osteotomy and negatively impact patient outcomes. A new study investigates a simple yet potentially powerful technique to address this issue: the addition of a protective screw.
Biomechanical Insights from Finite Element Analysis
To understand the mechanical implications of a protective screw, the researchers employed a sophisticated computational method known as Finite Element Analysis (FEA). This powerful tool is increasingly used in orthopedic biomechanics to simulate surgical procedures and predict how bone-implant constructs will respond to various forces. Unlike traditional in-vitro cadaveric testing, FEA allows for detailed, non-destructive analysis of stress and strain distribution within the bone.
The study created a three-dimensional model of a tibial sawbone to simulate a 10° opening wedge osteotomy. The model was then subjected to two primary loads:
- Axial Compression: Simulating the forces experienced during weight-bearing.
- Torsion: Simulating the twisting forces that occur during normal knee movement.
Crucially, the researchers compared the biomechanical behavior of the osteotomy with and without a protective screw intersecting the cutting plane at the theoretical lateral hinge.
The Impact of a Simple Screw
The findings from the FEA were conclusive and demonstrated a significant reduction in stress on the lateral hinge when the protective screw was in place. In the compression simulation, the area of the hinge subjected to high-level stress was dramatically reduced with the addition of the screw. Similarly, during the torsion simulation, the screw led to a notable decrease in the maximum stress value within the lateral hinge. This suggests that the screw acts as an internal buttress, distributing the applied loads and shielding the fragile hinge from fracture-inducing forces.
These results are particularly significant because a lateral hinge fracture can lead to a loss of the achieved angular correction, resulting in a poor functional outcome and potentially a need for further surgery. By enhancing the stability and strength of the osteotomy site, this simple technique could reduce the risk of this complication, leading to more predictable surgical results, faster rehabilitation, and improved long-term patient satisfaction.
A Promising Future for HTO
While the study’s findings are based on a computational model, they provide a strong biomechanical rationale for incorporating a protective screw into the MOWHTO surgical protocol. The authors themselves recommend that these findings be confirmed by further mechanical testing on physical specimens. If validated, this straightforward technical tip could become a standard practice, helping to make an already successful surgery even safer and more reliable for patients.
Professor Wilson’s Comments
“We were very impressed with the hinge wire concept. It was originally designed as a guiding pin within the patient-specific instrumentation, positioned at the very lower end of the device to act as a protective stop for the saw. Interestingly, when biomechanical testing was performed, it was found that this pin significantly increased the strength of the osteotomy at the hinge — an area recognised as a potential weak point and a common site for fracture, one of the known complications in both HTO and DFO surgery.
Kristian and I then considered how we could apply fundamental orthopaedic principles to address this issue of hinge fracture. The basic principle of stabilising two pieces of bone is to achieve direct compression, and this led us to revisit one of the earliest and most important innovations in osteotomy surgery — the “golden screw” concept, first used with the TomoFix plate. This design allowed controlled compression of the hinge area onto the plate to enhance stability.
Building on this principle, we thought — why not do what we always do in orthopaedics and go for direct compression? Rather than relying on a hinge wire, we could achieve this through a hinge screw. This idea gave birth to the hinge screw concept, which we subsequently applied to both tibial and femoral osteotomy surgery. The technique has since been adopted worldwide, and we are immensely proud of this development.
Together with our partners at Newclip Technics, we went on to design a dedicated hinge screw kit, which is now freely available and widely used in osteotomy procedures across the globe — helping surgeons achieve stronger, more stable constructs and reducing one of the most common complications of osteotomy surgery.”
For a comprehensive analysis of the full data and methodology, we encourage you to read the complete research paper:
Adding a Protective Screw Improves Hinge’s Axial and Torsional Stability in High Tibial Osteotomy
Sources used in report overview:
- https://pubmed.ncbi.nlm.nih.gov/32151903/
- https://www.researchgate.net/publication/339641209_Adding_a_protective_screw_improves_hinge’s_axial_and_torsional_stability_in_High_Tibial_Osteotomy
- https://www.researchgate.net/publication/348087448_A_supplemental_screw_enhances_the_biomechanical_stability_in_medial_open-wedge_high_tibial_osteotomy
- https://www.researchgate.net/publication/372076327_Lateral_hinge_fracture_in_medial_opening_wedge_high_tibial_osteotomy_a_narrative_review
- https://www.research-collection.ethz.ch/bitstreams/64032b91-cda6-4601-8bea-3914d8fe0f2e/download
