DOES THE PRESENCE OF SCREW-THREADS ACROSS AN ANKLE JOINT LOWER THE RATE OF FUSION AND THE OPTIMAL CHARACTERISTICS OF SCREW-THREAD LENGTH FOR ARTHROSCOPIC ANKLE ARTHRODESIS

Abstract

Introduction: Arthroscopic ankle arthrodesis is an effective treatment for end-stage arthritis.

A screw that has any thread across the fusion site can not offer any compression and may be postulated to lower the rate of fusion. Similarly, maximal screw thread in the target bone would optimise fixation.

This retrospective study calculates the ideal characteristics of a screw used for ankle arthrodesis, and assesses the correlation between the lack of compression and non-union.

Method: Fifty-one consecutive patients(102 screws) who have had arthroscopic ankle fusions were studied. Either AO(n=38) or ACE(n=64) screws were used. We calculated the screw-thread distance that crossed the talotibial(TT) or the subtalar(ST) joints on digitised images, and recorded the outcomes of the fusions.

Result: Of the 38 AO screws, only one (2.6%) had threads across the TT joint(thread-length into the joint = 1.2mm). None of the ST joints had been breached.

Of the 64 ACE screws, 8(12.5%) had threads across the TT joint, representing 7(21.9%) of all posterior screws and 1(3%) of all anterior screws in the study. The mean length of screw-threads into the TT joint was 2.1mm(range 0.53 to 4.06 mm). The ST joints were breached by 4(6.25%) screws(all posterior). The mean length of protrusion was 1.8mm(range 0.28 to 3.89mm). No screw thread crossed both TT and ST joints simultaneously.

No non-unions were recorded in either group.

Discussion: In our study the use of ACE screws for arthroscopic arthrodesis increases the risk of screw-threads breaching either the TT or ST joints when compared to the use of AO screws.

Conclusion: We conclude a thread length of 19–20mm would optimise fixation/compression. However no clinical correlation was demonstrated between reduced compression and non-union in this study.