30.1 Goals of Procedure

Cartilage defects involving a portion of the humeral head are a condition that may occur due to multiple causes, each of which poses a treatment challenge. A large portion of these osteochondral defects are caused by either avascular necrosis (AVN) or traumatic glenohumeral dislocation, both of which are relatively rare events.

AVN of the humeral head is the second most common site secondary to the femoral head. Frequently, this process is idiopathic, but may also be associated with steroid use, systemic lupus, chronic renal failure, or alcoholism. ¹ In the presence of AVN, the subchondral bone of the humeral head becomes necrotic and loses the mechanical integrity to support the cartilaginous surface. The cartilage than collapses, leaving a lesion in the humeral head, which causes pain and dysfunction. ¹

Traumatic glenohumeral dislocations, a majority of which are anterior, may result in a defect in the posterosuperior humeral head known as a Hill–Sachs lesion. This lesion is caused when that area of the head traumatically contacts the anteroinferior glenoid edge after the dislocation. ² A reverse Hill–Sachs lesion may occur through a similar mechanism in a posterior dislocation. In either event, a large enough defect can lead to recurrent instability by engaging with the glenoid edge and levering the head out of its articulation with the glenoid. While these lesions may not completely involve articulating cartilage of the joint surface, the presence of the defect leads to dysfunction through recurrent dislocation.

There is the possibility of limiting the pathology of these defects by replacing the defective surface of the humeral head with either osteochondral allograft or prosthetic replacement. The goal of either of these procedures is to reconstruct the native anatomy of the humeral head, while leaving the intact cartilage undisrupted. ^{3 , 4} Either reconstruction should fill the defect, be on the same level of the surrounding cartilage, and match the contour and radius of the surrounding head. While we recognize that other soft-tissue procedures can be performed for engaging Hill–Sachs lesions, this chapter will focus on partial arthroplasty with either allograft or prosthetic.

30.2 Advantages

Operative treatment options for partial surface humeral head lesions are limited and consist of either filling the defect or replacing the entire humeral head. There are multiple advantages involved in partially filling or replacing the defect rather than the whole cartilaginous surface. First, it allows the surgeon to maintain the normal anatomy of the humeral articular surface. When replacing the entire head, there is a risk of malalignment or incorrectly sizing the replacement. In a partial replacement, the natural inclination, version, and size are more easily maintained. Second, intact healthy cartilage is spared, allowing the patient to live with the greatest amount of their natural cartilage. Third, this minimalistic approach allows for greater reconstructive options in the future, which is ideal for younger patients. Finally, an advantage of an osteochondral allograft is the potential for bony healing and incorporation, but comes with the risk of not healing and resorption. A prosthetic partial replacement has the advantage of minimized wear and no risk of resorption with the risk of loss of fixation and loosening.

30.3 Indications

As mentioned earlier, the majority of indications for partial humeral head replacement are for osteonecrosis of the head and Hill–Sachs lesions. The size of the osteonecrotic lesion should be measured and be less than 40% of the entire surface area of the humeral head. ⁵ Hill–Sachs or reverse Hill–Sachs lesions are indicated for partial replacement if they are large enough to engage with the glenoid and create instability.

30.4 Preoperative Preparation/Positioning

As with any orthopaedic evaluation, a thorough history and physical examination should be obtained. Focus should be on a history of steroid use for systemic disease or, otherwise, sickle cell anemia, alcohol use, and renal disease. Additionally, a history of dislocation events including number of dislocations and trauma should be focused on if concern is more for Hill–Sachs lesions. A standard shoulder examination including inspection, palpation, range of motion, and special examinations focusing on pain during midrange for AVN or apprehension and relocation tests for instability. Finally, imaging should be obtained to corroborate the history and findings of the physical examination. Standard X-rays are valuable to obtain information on the native alignment of the humerus, presence of arthritic changes, and initial evaluation of defect location. Advanced axial imaging is also needed to completely evaluate the location and size of the lesion. For AVN lesions, MRI is the best study to evaluate the lesion, its effect on the cartilage, and to determine the size relative to the head. For Hill–Sachs lesions, CT imaging is preferred for more detailed bony evaluation of the size and location of the lesion.

At our institution, the vast majority of these lesions will be treated through an anterior deltopectoral approach. For posteriorly oriented Hill–Sachs lesions, a posterior approach can also be utilized. The location of the lesion and the comfort of the surgeon with the approach will determine the preoperative positioning. For the deltopectoral approach, we use a beach-chair position with the patient’s head elevated to approximately 30 degrees. If it is decided to perform a posterior approach, the patient is positioned in lateral decubitus with the operative arm draped free to allow free range of motion.

The choice of humeral head allograft versus prosthetic partial replacement should be made prior to the procedure; however, all options should be made available as backup for surgery. Indications for allograft are for the largest deepest lesions that are within the parameters of partial replacement. Additionally, allograft is reserved for younger, healthier patients with better bone stock for integration and also possesses a theoretical advantage of decreased glenoid wear. Partial replacement is generally reserved for the older end of the indicated population with the understood risk that there may be increased rate of glenoid wear.