SICOT e-Newsletter
Issue No. 63 - December 2013
Worldwide News
Comment by Mahmoud M. Abousayed
Kasr Al-Ainy Hospital, Cairo University, Egypt
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Screw fixation of medial malleolar fractures: A cadaveric biomechanical study challenging the current AO philosophy
The AO Foundation advocates the use of partially threaded lag screws in the fixation of fractures of the medial malleolus. However, their threads often bypass the radiodense physeal scar of the distal tibia, possibly failing to obtain more secure purchase and better compression of the fracture.
We therefore hypothesised that the partially threaded screws commonly used to fix a medial malleolar fracture often provide suboptimal compression as a result of bypassing the physeal scar, and proposed that better compression of the fracture may be achieved with shorter partially threaded screws or fully threaded screws whose threads engage the physeal scar. We analysed compression at the fracture site in human cadaver medial malleoli treated with either 30 mm or 45 mm long partially threaded screws or 45 mm fully threaded screws. The median compression at the fracture site achieved with 30 mm partially threaded screws (0.95 kg/cm² (interquartile range (IQR) 0.8 to 1.2) and 45 mm fully threaded screws (1.0 kg/cm² (IQR 0.7 to 2.8)) was significantly higher than that achieved with 45 mm partially threaded screws (0.6 kg/cm²(IQR 0.2 to 0.9)) (p = 0.04 and p < 0.001, respectively). The fully threaded screws and the 30 mm partially threaded screws were seen to engage the physeal scar under an image intensifier in each case. The results support the use of 30 mm partially threaded or 45 mm fully threaded screws that engage the physeal scar rather than longer partially threaded screws that do not. A 45 mm fully threaded screw may in practice offer additional benefit over 30 mm partially threaded screws in increasing the thread count in the denser paraphyseal region. Cite this article: Bone Joint J 2013;95-B:1662â6. |
Comment:
In this study the authors had challenged the traditional AO (Bible of Internal Fixation) Foundation principles of fixation of medial malleolar fractures. Unlike all other studies focusing on the design characteristics of the screw that optimise the pull-out strength and compressive force, they focused the site of purchase of the distal threads assuming that the physeal scar would provide a better purchase than the low density metaphyseal area proximal to it.
The study, which was done on 21 human cadavers with the same ankle fracture at the level of the tibial plafond, showed that significantly higher compression at the fracture site can be generated with the use of long (45mm) fully threaded screws or shorter (30mm) partially threaded screws compared with long (45mm) partially threaded screws. The main reason of this return to the fact that the long fully threaded screws and the short partially threaded screws engage the radiodense distal tibial physeal scar while the threads of the long partially threaded screws bypass it. This concludes that the radiodense distal tibial physeal scar clearly offers the best purchase for the threads of the screw and the most stable fixation.
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In our opinion, a few weak points, which need to be tackled by other papers in the future, include: the study was done on human cadavers not on patients, which are not subjected to shear or torsional forces; the study did not examine different types of medial malleolus fractures, e.g. suppination adduction type where medial malleolus is fractured vertically; the sample size of the study was too small to examine a relationship between compression at the fracture site and bone density. Lastly, a weak point, which the authors themselves mentioned, is the need for compression versus a good purchase in young patients, in which a fully threaded screw would act as positional screw only.
Finally in our opinion, this study will change the principles of fixation of medial malleolar fractures especially in osteoporotic bones.
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The study, which was done on 21 human cadavers with the same ankle fracture at the level of the tibial plafond, showed that significantly higher compression at the fracture site can be generated with the use of long (45mm) fully threaded screws or shorter (30mm) partially threaded screws compared with long (45mm) partially threaded screws. The main reason of this return to the fact that the long fully threaded screws and the short partially threaded screws engage the radiodense distal tibial physeal scar while the threads of the long partially threaded screws bypass it. This concludes that the radiodense distal tibial physeal scar clearly offers the best purchase for the threads of the screw and the most stable fixation.
| Screw used | Median compression/20 sec. | IQR (interquartile range) |
| 45mm fully threaded | 1.0 kg/cm² | 0.7-2.8 |
| 30mm partially threaded | 0.95 kg/cm² | 0.8-1.2 |
| 45mm partially threaded | 0.6 kg/cm² | 0.2-0.9 |
In our opinion, a few weak points, which need to be tackled by other papers in the future, include: the study was done on human cadavers not on patients, which are not subjected to shear or torsional forces; the study did not examine different types of medial malleolus fractures, e.g. suppination adduction type where medial malleolus is fractured vertically; the sample size of the study was too small to examine a relationship between compression at the fracture site and bone density. Lastly, a weak point, which the authors themselves mentioned, is the need for compression versus a good purchase in young patients, in which a fully threaded screw would act as positional screw only.
Finally in our opinion, this study will change the principles of fixation of medial malleolar fractures especially in osteoporotic bones.
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