Prosthetics Basics and Subtotal Amputation

The current dogma goes something like this: “animals do great on 3 legs.” This position advocates for total limb amputation when catastrophic injury (e.g., crushing and degloving) or pathology (e.g., neoplasia) arises. However, the short- and long-term structural consequences of a missing limb or limb segment are being recognized and defined in part through the efforts of pain management veterinarians (the American College of Veterinary Anesthesia and Analgesia and the International Veterinary Academy of Pain Management) sports medicine specialists (the American College of Veterinary Sports Medicine and Rehabilitation, and rehabilitation therapists (the American Association of Rehabilitation Veterinarians). As we begin to understand the biomechanics of normal quadruped locomotion, the implications when it is lost become clear. In terms of limb absence or total limb amputation, these include limited mobility and endurance, increased metabolic demand, weight gain, support limb break- down, chronic neck and back pain, and premature euthanasia (Figs 10 and 11). Because of these significant consequences, consideration must be made for the re-establishment of quadruped structure whenever possible.In human medicine, amputation at the hip for a catastrophic ankle injury would be unthinkable. Yet, this is standard of care in veterinary medicine. Why is this? In the past, prosthetics were not available and therefore concern for injury to the remaining limb segment was valid. Fortunately, prosthetics coupled with subtotal amputation, standard of practice in human medicine, are recent and successful developments in veterinary medicine. Given the consequences of limb loss in the short and long term, it seems appropriate to “contemplate before we amputate” an entire limb when only the distal segment is beyond salvage. Examples include neoplasia, trauma, and partial agenesis. Preservation of at least 50% of the radius or ulna or tibia or fibula allows ready application of a socket-based or ITAP prosthetic limb in species including dogs, cats, as well as others. Subtotal amputation is possible at nearly every level of distal joint as well as transtibial and transradial levels. The basic tenet is to preserve as much limb as possible while providing a tension-free closure. The ideal level of amputation for each injury, the best techniques, and the advantages or disadvantages of each level are still being defined. Regardless, the surgical techniques are simple, and complications are few.The biomechanics of the quadruped make design of prosthetic limbs challenging, but not insurmountable. The end goal is to provide a limb that allows as close to normal ambulation as possible. Angulation of the thoracic limb vs. pelvic limb, breed differences, as well as level of subtotal amputation must all be considered. Furthermore, these considerations are distinctly different from the biped human amputee. Detailed quadruped prosthesis biomechanics are beyond the scope of this article.[caption id="attachment_media-184" align="alignleft" width="234"]

Fig. 10. Severe carpal varus and digital supination secondary to left thoracic limb
total amputation.[/caption]Currently there are 2 types of prosthetic limbs available: socket based and ITAP. Socket-based prostheses have been used in humans for centuries and provide a socket within which the residual limb rests; an extension provides contact to the ground via some form of foot or paw. The key for socket-based prostheses is suspension and retention of the device on the residual limb. Improvements in materials, mechanical joints, and microprocessors have revolutionized these devices. ITAP provides an implanted endoprosthesis to which an exoprosthesis is attached. Surgery is required and the endoprosthesis is integrated into the bone and skin similar to the way an antler is attached a deer's head; no socket is required.Prosthetic limbs for animals are becoming available albeit with simpler models than those for humans. The advantage of socket- based prosthetic limbs is their relatively low cost, simplicity of application (no additional surgery required), and adaptability to many levels of limb loss from paw to midantebrachium or crus. The clear advantage of ITAP is direct skeletal integration of the exoprosthesis. This means there is no mechanical delay in gaiting because the exoprosthesis directly transmits forces to the skeleton via the endoprosthesis (implant). An additional advantage is less soft tissue trauma, an intermittent vexing although not intractable sequelae of socket prosthesis in humans and animals.[caption id="attachment_4749" align="alignright" width="313"]

Fig. 11. The spinal consequences of thoracic limb loss include chronic cervical,
thoracic, and lumbar spine repetitive strain as well as carpal joint collapse.[/caption]Rehabilitation is critical for the prosthesis patient human or animal. Control of the limb is reversed; top down rather than ground up control results in delayed feedback. Through rehabilitation, the prosthesis patient relearns proprioception, balance, gaiting at different speeds, and ambulation over varied terrain.