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Headless Compression Screw Fixation of Vertical Medial Malleolus Fractures is Superior to Unicortical Screw Fixation

August 29, 2018 | Surgery
Adam M.Wegner, MD, PhD ; Philip R. Wolinsky, MD ; Michael A. Robbins, BS ; Tanya C. Garcia, MS ; Sukanta Maitra, MD ; Derek F. Amanatullah, MD, PhD
Author and Disclosure Information

The authors report no actual or potential conflict of interest in relation to this article.

Acknowledgments: The authors would like to thank AO North America for the North American Resident Research award that helped to fund the synthetic sawbones required for this project. The authors would also like to thank DePuy Synthes and Acumed for supplying hardware for the internal fixation constructs.

Dr. Wegner and Dr. Maitra are Orthopaedic Surgery Residents, Dr. Wolinsky is a Professor of Orthopaedic Surgery, and Mr. Robbins is a Medical Student, Department of Orthopaedic Surgery, University of California Davis Medical Center, Sacramento, California. Ms. Garcia is a Lab Manager, JD Wheat Veterinary Orthopedic Research Laboratory, School of Veterinary Medicine, University of California Davis, Davis, California. Dr. Amanatullah is an Assistant Professor of Orthopaedic Surgery, Department of Orthopaedic Surgery, Stanford University, Redwood City, California.

Address correspondence to: Derek F. Amanatullah, MD, PhD, Department of Orthopaedic Surgery, Stanford Hospital and Clinics, 450 Broadway Street, Redwood City, CA 94063-6342 (tel, 650-723-2257; email, dfa@stanford.edu).

Adam M.Wegner, MD, PhD Philip R. Wolinsky, MD Michael A. Robbins, BS Tanya C. Garcia, MS Sukanta Maitra, MD Derek F. Amanatullah, MD, PhD . Headless Compression Screw Fixation of Vertical Medial Malleolus Fractures is Superior to Unicortical Screw Fixation. Am J Orthop. August 29, 2018

TAKE-HOME POINTS

  • Optimal fixation of vertical sheer ankle fractures is unknown.
  • Headless compression screws are stiffer than cancellous screws in offset axial load.
  • Headless compression screws have a higher load to failure than cancellous screws.
  • Headless compression screws may offer a soft tissue friendly fixation of method for vertical sheer ankle fractures.
  • These findings may not apply when subject to cyclic loads or in osteoporotic bone.

DISCUSSION

When subjected to offset axial load, we observed that the headless compression screw construct exhibited significantly increased stiffness and load to 2 mm of displacement compared with a parallel unicortical screw construct. The headless compression screw also demonstrated elastic recoil to almost 0 mm of displacement, which is well below the 2-mm displacement. 

We made reproducible fractures and fixation methods in synthetic distal tibiae, which feature less variability in size and quality than the cadaveric bone. Offset axial loading, rather than direct axial loading previously described by Amanatullah and colleagues,18 is the most physiologically relevant mode of force application to simulate the loading of the tauls onto the medial malleolus in the supination-adduction mechanism of injury.

The limitations of this study include the use of synthetic rather than cadaveric bone. Fourth-generation sawbones have been validated as possessing similar biomechanical properties as real bone.7,19 These results may also be inapplicable to osteoporotic bone, which would be significantly less dense than sawbones. This study is also an artificial situation designed to only test construct stiffness and load to clinical failure in a single mode of stress, offset axial loading and neglects other possible modes of force. This testing setup also disregards the structures surrounding the medial malleolus and tibia, including the talus, fibula, or soft tissue attachments, including the deltoid ligament and flexor retinaculum. These results are only relevant immediately after fixation and before bone healing occurs. We also tested the load to clinical failure rather than cyclic loading. Our testing more closely modeled a single traumatic force rather than the considerably smaller stresses that would be repeatedly exerted on the construct over several weeks after fixation in a clinical situation. This research is also not a clinical outcome study, rather, it suggests that headless compression screws are a viable, stronger, and possibly superior method for the initial fixation of vertical medial malleolar fractures.

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As the load is offset axial, the larger thread purchase of the headless compression screws may lead to increased pullout strength, possibly increasing headless compression screw construct stiffness. Also, the variable diameter of headless compression screw, which reaches up to 4.7 mm, would increase the stiffness of the construct compared with the diameter of the cancellous screws. The elasticity of the headless compression construct may be because screws are made of titanium rather than stainless steel. Such property and given that the screws are cannulated rather than solid may also play a role, although several studies have shown variable results for cannulated vs solid screws of the same diameter.20,21 The elastic section modulus of both screws would have to be calculated to determine their exact effect on fixation.

CONCLUSION

The headless compression screw construct was found to be stiffer and features a higher load to clinical failure than a parallel unicortical cancellous screw construct for fixation of vertical medial malleolus fractures. Although significantly increased cost occurs with this construct, the headless design may decrease soft tissue irritation, and the elastic recoil of the construct after displacement may decrease clinical failure rates of this fixation method. This condition would eliminate the need for revision surgeries and thus be a cost effective alternative overall.

This paper will be judged for the Resident Writer’s Award.

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