The 5-degree golden rule - An Introduction

The target tibial plateau (TPA) angle of 5 degrees for the TPLO procedure is so inherent to our everyday practice that it has become an unquestionable rule for almost two decades. Although I do not question that this target angle will serve most patients well, I wanted to understand the origin of the 5-degree golden rule and challenge its robustness.

The TPLO technique was first published by its inventors in 1993 (1). In this first publication, the authors suggest that levelling the tibial plateau would eliminate the cranial tibial thrust. By levelling, they meant rotating the tibial plateau to be perpendicular to the tibia's mechanical axis. This would achieve a TPA of 0 degrees.

Nevertheless, the same authors, five years later, in 1998, recommended a TPA of 5 degrees in the book Current Techniques in Small Animal Surgery, 4th edition (2). It seems that it was then that the 5-degree golden rule was established. But were there any studies to support this guideline before 1998? My literature research has tracked no such studies.

The first study on the optimal TPA target for the TPLO technique was published in 2001 in Veterinary Surgery by Warzee C et al. (3). The authors of this study concluded that a TPA of 6.5 degrees "may represent the optimal angle of tibial plateau rotation providing joint stability in cranial cruciate ligament-deficient stifles, while sparing the caudal cruciate ligament".

Based on the results from Warzee et al., a target TPA of 6.5 degrees is recommended in the book Veterinary Surgery, Small Animals (2nd edition) by Tobias. Nevertheless, in the same chapter, in a different paragraph (Preoperative Planning), the authors recommend using a chart to achieve a target TPA of 5 degrees. So, despite the evidence by Warzee et al., the 5-degree rule remains unbeatable.

Other studies have examined the mechanics of the stifle at TPA angles of 5 and 0 degrees (4,5), but the only research intended to find an optimal TPA angle was conducted by Warzee et al. For this reason, this will be the first study I will review in the next series of posts.

References

1. Slocum, B. and Slocum, T.D. (1993) Tibial plateau leveling osteotomy for repair of cranial cruciate ligament rupture in the canine. Veterinary Clinics of North America: Small Animal Practice, 23 (4), pp. 777-795.

2. Slocum, B. and Slocum, T.D. (1993) Tibial plateau leveling osteotomy for repair of cranial cruciate ligament rupture in the canine. Veterinary Clinics of North America: Small Animal Practice, 23 (4), pp. 777-795.

3. Warzee CC, Dejardin LM, Arnoczky SP et al (2001) Effect of titibal plateau leveling on cranial and caudal tibial thrusts in canine cranial cruciate-deficient stifles: an in vitro experimental study. Veterinary Surgery, 30, pp 278-286

4. Reif U, Hulse DA, Hauptman JG (2002) Effect of tibial plateau leveling on stability of the canine cruciate-deficient stifle joint: an in vitro study. Veterinary Surgery, 31, pp. 147-154.

5.  Shahar R, Milgram J (2006) Biomechanics of the tibial plateau leveling of the cranial cruciate-deficient stifle joint: A theoretical mode.. The Veterinary Surgery, 35, pp. 144-149.