It is 316L Stainless Steel—So It should be Good

Kenneth A. Johnson, MVSc, PhD, FACVSc, DACVS, DECVS

The veterinary orthopaedic implant marketplace is somewhat similar to the automobile industry. As soon as a producer brings a new and innovative product to the market, it is copied, either exactly or with subtle modifications, by other manufacturers. Occasionally such copying of veterinary orthopaedic implants is deemed to infringe intellectual property rights or patents and is challenged through the legal system. But this is often not the case.

Upon cursory comparison of “copy-brand” implants with the original product, it is often impossible to spot any differences between them, particularly in parameters like stiffness, fatigue life, and corrosion resistance. Moreover, unlike the practice of human orthopaedic surgery, where every new orthopaedic implant must pass the scrutiny of a regulatory process, such a requirement for veterinary orthopaedic implants does not exist. This creates a dilemma for the conscientious veterinary surgeon when choosing appropriate implants to use in their surgical practice. There are very few publications describing independent, objective head-to-head biomechanical or metallurgical comparisons of veterinary orthopaedic implants from different manufacturers.

The easiest biomechanical methodology by which to compare similar implants is quasi-static loading to obtain values for stiffness and load to failure in bending, torsion, or a combination of both.[1] [2]

Biomechanical testing of plate–bone (cadaveric or synthetic) constructs with cyclic loading is slightly more relevant to the in vivo situation for our patients. The study published in this issue of the Journal, comparing three locking compression plates in cyclic torsional loading reports some significant differences, as well as similarities between the implants from three different manufacturers.[3] This is very interesting, but as the authors point out, readers should appreciate the limitations of these data.[3] As this study used bone models for ex vivo testing in torsion alone, the surgeon cannot know if the fatigue life of all three constructs will be adequate for fracture healing to occur in all clinical patients. Therefore, these findings[3] are only the beginning of the story—not the end. We need to understand the reasons behind the differences between the performances of the three locking compression plates.

Apart from geometric differences in locking compression plates, such as plate thread-hole shape and dimensions, the biomechanical performance of 316L stainless steel implants can be affected by production methods and cold working. One might assume that the production of 316L stainless steel used in veterinary orthopaedic implants would be highly standardized. But recall there is no regulatory process to guarantee this assumption.

An important person whose work helped refine the 316L specification of stainless-steel orthopaedic implants was Dr. Ortrun Pohler. Dr. Pohler was a metallurgist who was employed for her whole career, starting in 1957, by the Straumann Institute in Waldenburg, Switzerland.[4] The Straumann Institute was contracted by the Arbeitsgemeinschaft fűr Osteosynthesefragen (AO) to produce the AO Instrumentarium.[4] Dr Pohler found that the early orthopaedic implants (not of the 316L standard) were prone to metal fatigue cracking and breakage due to corrosion pits caused by longitudinal “slag” inclusions. Her work refined AO implant 316L stainless steel standards and ensured that cold-worked plates were not sensitive to stress corrosion cracking, or pitting corrosion. This standard for AO implants set at the Straumann Institute became a global standard for AO implants worldwide.[4] [5] Most importantly, all AO veterinary orthopaedic implants produced by the current manufacturer (De Puy Synthes) are held to the same standards as human orthopaedic implants.

Dr. Ortrun Pohler, courtesy of AO Foundation

Unfortunately, some other commercially available veterinary orthopaedic implants that were claimed to be 316L stainless steel have been found to not meet this standard and are thus susceptible to corrosion.[6] A challenge for all producers that market veterinary orthopaedic implants should also be to provide evidence that their implant material meets the 316L stainless steel standards set and maintained by AO.

Publication History

Article published online:
14 January 2025

• References

• 1 Blake CA, Boudrieau RJ, Torrance BS. et al. Single cycle to failure in bending of three standard and five locking plates and plate constructs. Vet Comp Orthop Traumatol 2011; 24 (06) 408-417
• 2 Cabassu JB, Kowaleski MP, Skorinko JK, Blake CA, Gaudette GR, Boudrieau RJ. Single cycle to failure in torsion of three standard and five locking plate constructs. Vet Comp Orthop Traumatol 2011; 24 (06) 418-425
• 3 Lai LH, James DR, Appleyard RC, Cadman J. Biomechanical comparison of three locking compression plate constructs from three manufacturers under cyclic torsional loading in a fracture gap model. Vet Comp Orthop Traumatol 2024; (e-pub ahead of print)
• 4 Jeannet J-P. Leading a Surgical Revolution. The AO Foundation-Social Entrepreneurs in the Treatment of Bone Trauma. Springer Nature Switzerland AG; 2019: 121-128
• 5 Disegi JA, Eschbach L. Stainless steel in bone surgery. Injury 2000; 31 (Suppl. 04) 2-6
• 6 McCartney W, Liegey A, MacDonald B, Comiskey D, Galvin E. Carbon composition analysis of 12 selected stainless steel veterinary orthopaedic implants: a preliminary report. Vet Rec 2013; 172 (03) 71