Scientific, Technologic, Surgical, and Prosthetic Advances in Pediatric Limb Deficiency

Michael Schmitz MD, FAAOS

Rebecca Hernandez CPO, LPO

Dr. Schmitz or an immediate family member serves as a paid consultant to or is an employee of Orthofix, Inc., Orthopediatrics, and Stryker and serves as a board member, owner, officer, or committee member of Pediatric Orthopaedic Society of North America and Scoliosis Research Society. Neither Rebecca Hernandez nor any immediate family member has received anything of value from or has stock or stock options held in a commercial company or institution related directly or indirectly to the subject of this chapter.

ABSTRACT

Limb deficiency in children often requires lifelong adaptive measures to improve function, decrease morbidity, and allow for proper development. Targeted muscle reinnervation and regenerative peripheral nerve interfaces are techniques designed to redirect efferent neural impulses to improve both biologic and prosthetic function and decrease residual limb pain. Osseointegration is a technique that links a prosthetic device directly to the appendicular skeleton. Allotransplantation of limbs shows benefit in the upper extremity, however at a cost of the need for lifetime immunosuppression. Refinements in indications, implants, and techniques may lead to use in younger patients. Technologic advancements in fabrication, component modularity and miniaturization, and microprocessor control have improved prosthetic function and durability.

Introduction

Treatment options for pediatric limb deficiency would ideally offer biologic replacement or animated mechanical prostheses that could mimic muscular, neurologic, and skeletally supportive biologic function. Currently, no options can do so without attendant morbidity or functional compromise. However, advances in transplantation science, surgical tissue transfer, mechanical design, fabrication, miniaturization, and microprocessors have led to improvements in both biologic and mechanical function.

Scientific, Technologic, and Surgical Advances

Limb deficiency in children often requires lifelong adaptive measures to improve function, decrease morbidity, and allow for proper development.¹ The goal of limb deficiency treatment is to create a limb facsimile that can functionally replace the deficiency. The ideal replacement would provide afferent tactile, vibratory, and positional sense to the wearer; be able to interpret efferent motor control; translate the efferent motor impulses into physical action; provide structural support; remain pain free; and promote normal function with a low biologic complication rate and high durability. Current prosthetic devices and limb salvage procedures are limited by the bioprosthetic interface, lack of afferent sensation, limited motor control, and componentry unable to replicate biologic function. Surgical techniques create residual limbs with altered anatomy and severed nerves that can become painful neuromas. Developmental advancement, longitudinal limb growth, and elevated activity levels are three factors that make treatment of pediatric patients with limb deficiency more challenging. Refinements in surgical techniques, prosthetic design and fabrication, and rehabilitation can lead to less morbidity and greater function.

Targeted muscle reinnervation (TMR) and regenerative peripheral nerve interfaces (RPNIs) are techniques designed to redirect efferent neural impulses.²^,³ TMR transplants functioning nerves to denervated tissue, whereas RPNI transfers denervated free muscle patch grafts to transected nerves. Both techniques were originally designed to create bioamplifiers of efferent nerve impulses to improve control of myoelectric prostheses and incidentally were found to decrease both phantom limb pain and residual limb pain.⁴^,⁵^,⁶^,⁷^,⁸

Phantom limb pain should be distinguished from residual limb pain in that phantom limb pain refers to central sensation for a body part not present and residual limb pain to pain in the residual limb.⁹ Phantom limb pain has a reported prevalence of 7% to 100% and can occur regardless of the etiology of limb loss and persist for an extended period of time.¹⁰^,¹¹^,¹² Amputation etiology varies the prevalence, with malignancy the highest
(48% to 90%), trauma highly variable (12% to 83%), and congenital limb loss the lowest (4% to 20%).¹⁰

FIGURE 1 Schematic illustration shows targeted muscle reinnervation. A, Transection of primary nerve. B, Isolation of neurovascular supply to neighboring muscle. C, Transection of one discrete motor supplying neighboring muscle, leading to selective muscle deinnervation. D, Primary nerve to muscle motor nerve coaptation. E, Eventual reinnervation of neighboring muscle with fascicles from primary nerve. (Reproduced with permission from Herr HM, Clites TR, Srinivasan S, et al: Reinventing extremity amputation in the era of functional limb restoration. Ann Surg 2021;273[2]:269-279.)

Pain in the residual limb or phantom limb pain can significantly alter functional outcomes.¹³

TMR (Figure 1) transfers information from larger diameter proximal nerves severed from their muscle effectors to new muscle effectors that can serve as effective electromyographic (EMG) signal generators for receptors in myoelectric prostheses.¹⁴ The large-diameter peripheral nerves can be coapted to smaller nerves close to the entry into vascularized native muscle either primarily or after neuroma excision.¹⁵ The coaptation coordinates the otherwise neuroma-generating regenerative nerve fibers and decreases the incidence of both phantom and residual limb pain.⁷^,¹⁶^,¹⁷

RPNIs (Figure 2) are nonvascularized free muscle grafts coapted to a distal nerve ending, creating an EMG-generating end plate for the neurosignal.¹⁸ RPNI offers the ability to create multiple EMG generators for prosthetic control after reinnervation without sacrificing native vascularized muscles. RPNI inhibits neuroma formation by providing a denervated biologic target for both sensory and motor nerve ingrowth.¹⁹ Reinnervation of the new target is effective in both preventing and managing neuroma-mediated pain.⁸

The traditional method used to attach both an upper and lower limb prosthesis to a limb requires a bioprosthetic interface of a custom-designed socket. An excellent fit is required to transmit skeletal forces to the end component of the prosthesis. Fit is compromised by volume changes in the limb, dynamic soft tissue, and diaphoresis, which can lead to shear on the skin, pain, and skin breakdown, decreasing quality of life and function.²⁰

Osseointegration is a term coined by Brånemark after he found that bone integrated into titanium matrices, during a study of bone flow in rabbit models.²¹ Osseointegration refers to direct structural connection between bone and a metallic implant. It has been used successfully in dental implants, maxillofacial reconstruction, and total joint arthroplasties with great success. In some parts of the world, it is an established treatment option for select patients intolerant of traditional sockets.²²

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