Having been introduced more than 25 years ago by Barclay Slocum, the tibial plateau levelling osteotomy (TPLO) procedure was both revolutionary and controversial, yet it has stood the “test of time” while undergoing numerous modifications. The original Slocum TPLO plate was a cast stainless steel plate that had a triad of “compression” holes in the proximal part of the plate. This plate needed to be bent and twisted intra-operatively to match the contour of the proximal tibial metaphysis, and it was usually applied with cortical screws. However, it became apparent that both the material properties of this plate and its design features could be improved.
Introduction of the AO Locking Compression Plate in early 2000 was a critical impetus for the development of a procedure-specific, anatomically precontoured locking compression plate for the TPLO procedure in dogs.[1] Some of the major advantages of the AO TPLO plate were the capacity to maintain alignment of the osteotomy and the tibial plateau position during the insertion of locking screws, abolition of any requirement for intra-operative plate contouring, directed fixed angle locking screws (to avoid articular penetration), and greater construct stability compared with plates fixed with cortical screws. Insertion of a cortical load screw in the Combi hole of this locking plate generated interfragmentary compression across the more distal transverse part of the curvilinear tibial osteotomy. However, this is often accompanied by loss of compression and widening of the curvilinear osteotomy more proximally, just caudal to the tibial tuberosity.
An in vitro study published in this issue of the journal aimed to record the pressures generated at the interface of the TPLO using a pressure-sensitive film.[2] The application of point-to-point bone holding forceps across the TPLO in combination with insertion of a load screw distally in the compression hole of the plate provided more even interfragmentary compression within the tibial osteotomy, in comparison to other combinations of techniques.[2] These findings support the use of point-to-point forceps during the application of a TPLO plate with compression. Clinical studies in humans who underwent compression knee arthrodesis found that maintenance of interfragmentary compression at the osteotomy resulted in primary bone healing with woven and trabecular bone within 4 weeks.[3] [4] However, continued interfragmentary stability across a tibial osteotomy and primary bone healing relies on both the on-going maintenance of interfragmentary compression and friction between the cut bone surfaces, for this to occur. Loss of construct stability due to implant loosening, for example, can compromise healing. Readers of this in vitro study[1] should appreciate that the pressures recorded by the pressure-sensitive film are the maximal pressures at a single time point during the “surgical” procedure. We do not know if they are maintained in the postoperative period. Also, it is difficult to determine the magnitude of compression that needs to be generated by load screws or other instruments to produce the pressures within the TPLO, as recorded in this study.[2] These are interesting questions for future studies.
