TPLO to a TPA of 5 degrees - Reif et al.

This is the second post in a series investigating the reasons why the number 5 is so well-embedded in the TPLO literature.

Effect of Tibial Plateau Leveling on Stability of the Canine Cranial Cruciate-Deficient Stifle Joint: In Vitro Study

Ullrich Reif, Donald A Hulse, Joe G Hauptman

The primary objective of the study was to assess the caudal tibial thrust generated after a TPLO procedure, specifically in relation to a tibial plateau angle (TPA) of 5 degrees.

The researchers used six limbs of dogs that were euthanised for reasons unrelated to the study with a mean body weight of 30 kg. The limbs were mounted on a device that could induce compression between the femur and tibia, with the stifle joint placed at 60 degrees of flexion. The proximal tibial segment was secured in a circular frame mimicking the tibial osteotomy and could be rotated to change the TPA to a desired angle. The cranial cruciate ligament was transected while the tibia was compressed against the femur, inducing a mean cranial tibial translation of 14 mm. The compressive load was removed, and the TPA was changed to 5 degrees before the load was re-introduced. This resulted in a mean caudal tibial translation of 2 mm. A cranially-directed force was applied to the tibia to induce a cranial translation. The force was named cranial thrust, and its magnitude was gradually increased until a cranial tibial translation was achieved. It was assumed that the only opposing force to the cranial thrust was a caudal thrust, which was induced by the TPLO and was also the reason for the caudal tibial translation after the TPLO. Friction between the bones was considered negligible, based on a 1976 study. The higher the cranial thrust that had to be achieved to induce a cranial tibial translation, the higher the caudal thrust was assumed to be.

The researchers concluded that a TPLO to a TPA of 5 degrees induced a caudal tibial thrust in all limbs, and the thrust was proportional to the compressive axial load between the tibia and the femur. The fact that a gradual increase in the axial load resulted in a gradual increase in the caudal thrust suggests that a TPA of 5 degrees resulted in a tibial plateau inclination, such that it predisposed the tibia to slide caudally relative to the femur.

Based on the results of this study, the authors concluded that a TPA target of 5 degrees is reasonable, but it will induce a caudal tibial thrust. A smaller TPA will theoretically result in an increase of the caudal tibial thrust and in a potential injury of the caudal cruciate ligament.

The authors did not test different TPA angles to find an angle that would mitigate the cranial tibial thrust while minimising the caudal thrust. Their objective was to examine the effect of a TPA to 5 degrees on the caudal cruciate ligament, assuming that this is the main structure opposing a caudal tibial thrust.

Another less significant limitation is that the authors considered the friction between the femur and tibia to be negligible. However, even if a small amount of friction were taken into consideration, one would not expect this to have significantly changed the conclusions.

In my opinion, this is a well-designed, well-executed and well-discussed study. It suggests that a TPA close to 5 degrees was ideal for most limbs used. An induction of a caudal tibial thrust seemed unavoidable if one wanted to err to the side of caution. The magnitude of the caudal thrust was proportional to the magnitude of the compressive load between the femur and tibia.