Instability After Unlinked Total Elbow Arthroplasty

Introduction

The incidence of instability following an unlinked total elbow arthroplasty (TEA) has been reported to range from 0% to 13%. The concern about instability and the challenges in treatment have resulted in a trend for surgeons to use a linked implant when performing elbow arthroplasty. Many factors must be considered in assessing the cause and in preventing instability following elbow arthroplasty. These include the implant design, the surgical approach, surgical factors, patient factors, and postoperative management.

An understanding of the normal anatomic and biomechanical features of the elbow is essential when one is considering the prevention, causes, and treatment of unstable unlinked elbow arthroplasty. As discussed in Chapters 3 and 86 , elbow stability is provided by static constraints and dynamically by the muscles. The static constraints include the shape of the articulation itself as well as the capsule and the medial and lateral collateral ligaments. The tendons provide important additional stability that augments that afforded by the ligaments. The muscles that pass across the joint provide compressive forces to the articulation and have been demonstrated to play an important role in stabilizing the elbow. The stability of the native elbow is provided by a complex interplay between these bones, ligaments, tendons, and muscles. Disruption, or failure to heal, of one or more of these structures is an important cause of elbow instability. The three primary constraints to elbow instability include the ulnohumeral articulation, the anterior band of the medial collateral ligament, and the ulnar part of the lateral collateral ligament (also known as the “lateral ulnar collateral ligament”). The secondary constraints include the radiohumeral articulation, the common flexor and extensor tendon origins, and the capsule. Collectively these constraints are important to consider when one is performing an unlinked elbow arthroplasty to avoid the development of postoperative instability.

Classification of Total Elbow Arthroplasty Instability

Instability following TEA can be classified as early or late. Dislocation in the perioperative period tends to be due to either a complication of anesthesia (muscle co-contraction while awakening) or component malpositioning. In the early postoperative period (up to 6 weeks), disruption of the triceps repair or one or both of the collateral ligaments is usually responsible for instability. A very important factor is repetitive varus stress caused by the force of gravity, experienced by the elbow while the forearm is being moved in the horizontal or inclined plane. This is very difficult for patients to avoid, as it is intrinsic to activities of daily living. Repeated varus stress causes attenuation of the lateral collateral ligament and the development of posterolateral rotatory instability.

Late instability (after 6 weeks) is usually due to trauma, polyethylene wear, or attrition of a collateral ligament. Limb malalignment and component malposition may both contribute to the development of polyethylene wear.

Etiology

The common causes of instability following unlinked TEA include the implant design, the surgical approach, surgical factors, patient factors, and postoperative management.

The Implant Design

The design of a TEA implant has important effects on elbow stability. The intrinsic constraint of some unlinked implants does not replicate the stability provided by the normal elbow. Most unlinked devices such as the capitellocondylar, Pritchard ERS, Kudo, and iBP devices provide little intrinsic articular constraint relative to the native elbow, principally due to their shallow articular contours and reduced radii of curvature. Other unlinked designs, such as the Souter and Sorbie designs, had a much deeper groove in the humeroulnar articulation, affording greater stability. Another approach, used in the Latitude EV, uses an extended coronoid and a temporary ligament to prevent instability. Some implants incorporate a radial head prosthesis as part of their design, which should provide improved valgus and rotational stability of the elbow. However, if a radial head component is inserted in the incorrect alignment or position, it will have a propensity to subluxate or dislocate.

The normal elbow joint has an axis of flexion-extension that approximates that of a loose hinge. Restoration of this axis is essential to ensure soft tissue balance around the elbow following TEA. Challenges in identifying this axis and the difficulty in reproducing it with the available surgical instrumentation are likely an important cause of instability and wear. Finally, the radial head, if it is retained or replaced, must articulate concentrically with the capitellum in all positions of flexion and forearm rotation. Recent advances in instrumentation to correctly position components to replicate the axis of motion as well as the incorporation of bipolar radial head implants, which can accommodate for mild malpositioning, may improve the success rate of unlinked TEA.

The Surgical Approach

The surgical approach used for placement of an unlinked total elbow implant has important implications for the subsequent stability of the elbow. Many unlinked implants were historically inserted through a lateral surgical approach, which divides or detaches the lateral collateral ligament from the lateral epicondyle. An inadequate or failed repair of the lateral collateral ligament will result in posterolateral rotatory and varus instability. Surgical approaches that divide the medial collateral ligament may cause posteromedial or valgus instability in the setting of an inadequate or failed repair. All total elbow implants require sectioning of one or both of the collateral ligaments to allow implant placement. Newer devices provide securer techniques of ligament fixation that may reduce the incidence of instability following unlinked TEA.

One of the most important factors contributing to postoperative instability is the integrity of the triceps tendon, which is detached or reflected in many approaches. The triceps is an important dynamic stabilizer; a failure to heal causes posteromedial instability leading to attenuation of the medial collateral ligament. Surgical approaches that preserve the triceps integrity such as the paratricipital and lateral paraolecranon approach may reduce the incidence of instability following an unlinked TEA.

The Surgeon

Achieving stability with an unlinked implant is predicated on maintaining balanced forces across the elbow after it has been replaced. This is most reliably accomplished by an accurate restoration of the flexion-extension axis and proper repair and healing of the soft tissues. Malpositioning of the components will result in an abnormal location of the implant axis and hence incorrect ligamentous and muscular tension across the elbow. This will lead to articular maltracking, instability, and an increase in joint contact forces that ultimately causes implant wear. Abnormal restoration of the flexion-extension axis alters the muscle moment arms and thereby influences the dynamic stability and loading of the elbow. Even if a surgeon anticipates being able to use an unlinked implant design, it is important to understand that circumstances encountered at surgery may make a linked device more appropriate. It is important that a linkable device always be available when one is performing an unlinked TEA. This requirement has led to the development of convertible devices that can be used in both an unlinked mode and a linked mode.

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