The transhumeral socket interface presents several unique prosthetic challenges.¹ As with other levels of prosthetic interface design, it must provide adequate proximal musculoskeletal stability while managing the distal volume of the residual limb. These objectives must be accomplished even though the transhumeral prosthesis is suspended from a highly mobile proximal skeletal joint, with its own weight distracting the tissue distally. The triaxial stability and coupling of the interface to the residual limb is further influenced by the interaction of the upper limb harness design and by the choice of control system used. For example, body-powered systems with laterally mounted control cables may inadvertently pull the interface into external rotation if the socket is loose or does not have adequate posterior proximal support. In many instances, an otherwise well-made interface may not provide adequate comfort or suspension if the harness is not fitted well enough to address these functional suspension needs.

Similar to the transfemoral level, where the prosthetic socket provides volumetric containment of mobile soft tissue surrounding the relatively narrow femoral shaft, prostheses at the transhumeral level encompass the tissues surrounding the shaft of a humerus that is generally too narrow to provide the distal skeletal substructure needed to fully stabilize and maintain the position of the socket. As a result, both body-powered control activation and external loads create disruptive force couples within the socket that must be anticipated and managed. In the sagittal plane, the interface has the tendency to be pulled into extension as loads on the forearm cause the distal socket to rotate posteriorly relative to the residual limb. This places additional localized loads on the anterior distal area of the limb. In the frontal plane, patients with a high degree of glenohumeral abduction may experience increased lateral-distal loading if the arm is not properly aligned. In the absence of adequate soft tissue, these areas can be vulnerable to painful distal socket pressures. Alternatively, when treating patients with excessive redundant tissue, management of this tissue is important because the rigid skeletal structures are deeper and more difficult to load. The various lengths of transhumeral amputation provide additional challenges, with longer amputations requiring accommodation of the humeral condyles, and proximal-level amputations requiring greater proximal loading.

The prosthetist may be further challenged by a lack of any widely accepted, consistent clinical protocols for impression taking and modification techniques. In many instances, because of the relative rarity of transhumeral amputations, the clinician may not have had sufficient experience to have a clinical reference for managing these patients successfully and may lack self-efficacy.

All of these factors combine to make the transhumeral interface design challenging to manage and may contribute to low prosthesis acceptance rates (range, 27% to 61%) in individuals treated by practitioners unfamiliar with the transhumeral fitting level.^2,3,4,5 As with other levels of upper limb involvement, ultimate acceptance of prosthetic
use by transhumeral amputees is based on the ability to achieve their desired functional goals within their individual comfort tolerance. It is critical that prosthetists are aware of the process, components, concepts, and expected outcome for each prosthesis to ensure that their patients have the best chance of success.⁶

Although this chapter focuses on the prosthetic management of transhumeral amputations, related amputations are also briefly described. In certain instances of limb paralysis, such as a brachial plexus injury, patients may elect transhumeral amputation and fusion of the paralyzed glenohumeral joint, with 20° abduction, 30° flexion, and 40° internal rotation.⁷ In this elective amputation, all prosthetic elbow componentry can be accommodated if the humerus is amputated 100 mm (3.94 inches) proximal to the tip of the olecranon.^8,9 Although amputation offers a more functional solution than a flail arm, the decision to amputate is very difficult and amputation must be performed with great sensitivity because it involves the removal of an arm that appears normal.

An elbow disarticulation (through-elbow amputation) has several advantages, including maximizing the length of the mechanical lever arm, minimizing disruption to soft tissues, providing a load-tolerant distal end, and permitting distal supracondylar suspension.^10,11 However, the major disadvantage is that the long length of the prosthesis precludes the use of distally mounted prosthetic elbow mechanisms. Rather, this amputation level necessitates the use of elbow hinges that are laminated outside of the interface (Figure 1). Cosmetically, this increases the distal mediolateral dimension at the elbow joint. Functionally, it restricts the number of componentry options and shortens the prosthetic forearm.

Pediatric amputation can become problematic because of pointed bony overgrowth that may emerge from the cut end of the bone.¹² Elbow disarticulation is often used in this cohort because it reduces bony overgrowth by preserving the epiphyseal growth plates. As the child ages, growth of the ipsilateral humerus can be surgically restricted to shorten the arm over time so that the length discrepancy is not noticeable in adulthood. This creates a load-tolerant residual limb capable of self-suspension at a transhumeral limb length.

Less frequently used variants of transhumeral amputation have also been described with the intent of improving the rotational stability of the socket over the residual limb.^9,10,12,13 These include an osteotomy procedure described by de Luccia and Marino¹⁴ in which a bony section of the diaphysis is removed to place the humeral epicondyles more proximally (Figure 2). In another variation, Marquardt and Neff¹² described an angulation osteotomy that fixes the distal humeral shaft length at 45° (Figure 3). This procedure
is most often used to treat patients with bilateral amputation or those desiring a more secure coupling with the transhumeral interface.^9,10,13

Osseointegration has begun to emerge as a possible alternative, eliminating the need for traditional prosthetic interfaces. In this surgical approach a distal titanium abutment is secured to an implant that has been screwed or press-fit in the remaining humeral diaphysis. The external prosthesis must incorporate a proximal attachment that receives the abutment. Transhumeral osseointegration eliminates the need for a harness for axial support, allowing full glenohumeral rotation and a greater range of abduction than standard prosthetic fitting methods (Figure 4).

It is commonly accepted that early prosthetic fitting results in greater acceptance of an upper limb prosthesis. The 30 days after surgery are often referred to as the golden period for prosthetic fitting.¹⁵ It is thought that if fitting occurs beyond this period, the patient will have adapted to some degree, becoming reliant on unilateral activation strategies.¹⁵ Early management of the amputation results in volume reduction and pain attenuation by enclosing the residual limb in a more rigid dressing or a flexible liner.¹⁶ Early prosthetic fitting also may have a psychological benefit because the patient can begin to incorporate the proprioception or kinesthetic awareness of the prosthesis into their body image.

Elastic shrinker socks or bandages can be used to initially shape and reduce the distal soft-tissue volume. Subsequently, a basic upper limb prosthesis can be constructed of endoskeletal componentry and attached to a rigid dressing or preparatory socket to begin training in prosthesis control. After the shape and volume of the distal limb stabilize, a more definitive interface can be made (Figure 5). As the limb undergoes volumetric changes, the use of an adjustable harness will assist in maintaining suspension.

During the postoperative phase, the rehabilitation team should meet to establish immediate, short-term, and long-term goals.^17,18 The preparatory prosthesis allows the patient to become accustomed to the loading characteristics, control movements, weight, and operation of a prosthesis.^17,18 The prosthetist should involve the patient and their support group in all phases of prosthesis development. The patient who is informed about recommendations and who actively participates in decisions regarding their prosthesis will typically establish a greater sense of ownership and dedication to the process.^17,18

The value of immediate psychological counseling and peer visits during the early postoperative phase should not be underestimated because the upper limb plays a vital role in function and human social interaction. A peer who has experienced upper limb loss can help the new amputee establish realistic expectations and prepare for future challenges that may aid in long-term prosthesis acceptance and use.¹⁸

Most transhumeral amputations involve the use of anterior-posterior flaps for closure, with a myodesis of the biceps and triceps muscles to the distal humerus to preserve stability and maintain alignment.¹⁰ Additional myoplasty is performed to preserve the soft-tissue padding and muscular balance of the residual limb. Myoplasty provides good distal padding, but it may make it difficult for the patient to differentiate the independent myoelectric signals during initial training.

Although the muscle bellies of the biceps and triceps are initially in the original longitudinal physiologic position, there is a tendency for them to migrate medially, which alters the position of the electromyographic (EMG) sites as the limb matures. It is important to recheck and adjust EMG sites to maintain correct positioning. If the muscle bellies release from the myodesis or myoplasty, muscle bunching may occur, with the muscle belly migrating proximally and medially during contraction. This can create problems in volume management and the placement of myoelectrodes as the muscle dynamically contracts. This internal movement can also cause release of the proximal seal within suction sockets, which allows air to enter into the socket and eliminates the negative-pressure environment necessary for suspension.

Because most of the muscle structures are left intact, these concerns are less common with elbow disarticulation. However, some surgical reduction of distal soft-tissue bulk may be preferred because it allows the transverse geometry of the distal humerus to provide greater suspension and rotational control. Excessive distal redundant tissue can prevent a tight fit and impede control of the prosthesis.⁸

Muscle transfers and targeted muscle reinnervation techniques can be used to provide additional EMG sites for external power activation. Transfer of an innervated latissimus dorsi, a gracilis, or a pectoralis muscle can be used to create useful EMG sites if none are available^19,20,21 (Figure 6). Targeted muscle reinnervation repositions existing nerves to the remaining muscle groups that have been separated. Some patients who have undergone such procedures have achieved surprising levels of control complexity in combination with sophisticated pattern recognition control systems.²²