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Correcting torsional deformities with Ilizarov's apparatus
J E Herzenberg1, J D Smith, D Paley
1University of Maryland Medical School, Baltimore.
Clinical Orthopaedics and Related Research
|May 1, 1994
Summary
Circular fixator use in limb lengthening can cause rotational deformities. A new trigonometric equation predicts and helps manage unintended bone translation during derotation, improving surgical outcomes.
Area of Science:
- Orthopedic surgery
- Biomechanical engineering
Background:
- Limb lengthening using circular fixators (e.g., Ilizarov frame) is common.
- Rotational deformities frequently accompany limb lengthening and require correction.
- Eccentric bone positioning in fixator rings can cause secondary translation during derotation.
Purpose of the Study:
- To present a trigonometric equation for predicting inadvertent bone translation during derotation in limb lengthening.
- To provide surgeons with a tool to anticipate and manage translation deformities.
Main Methods:
- Derivation of a trigonometric equation: t = (r sin theta)/sin (90 -theta/2).
- Approximation of the equation for derotation angles < 45 degrees: t = 0.017 theta r.
- Analysis of factors influencing translation: distance from ring center to bone edge (r) and derotation angle (theta).
Main Results:
- The derived equation accurately predicts the amount of translational deformity (t) induced by derotation.
- The simplified equation provides a practical approximation for common clinical scenarios.
- Surgeons can use these calculations to plan corrective strategies.
Conclusions:
- Understanding the relationship between derotation and translation is crucial for managing limb lengthening deformities.
- Pre-operative calculation of induced translation allows for proactive surgical planning.
- Options include using modified frames or subsequent translation correction to optimize results.