The Loose Glenoid: A Problem of Consequence

Anatomic Considerations 

Preoperative evaluation is important for appropriate planning of glenoid implantation. In a nonarthritic shoulder, glenoid vault volume and surface area are small, measuring approximately 12 ± 5 cm3 and 8 ± 4 cm2.6 Osteoarthritis typically causes cartilage and bone erosion, which can be asymmetric. Eccentric wear can make initial implantation and future retraction of a loose implant challenging.

Shoulder radiographs in the true anterior–posterior plane in neutral, internal, and external rotation should be used for initial evaluation of the patient and can identify the presence of glenohumeral disease. The lateral film, or outlet view, is useful to define acromial morphology. The best radiograph for evaluation of the glenoid is the axillary lateral (Fig. 1). In addition, a computed tomography (CT) or magnetic resonance imagery (MRI) scan can evaluate glenoid morphology more accurately (Fig. 2A and B). While not always necessary, the additional scans can evaluate glenoid retroversion, humeral subluxation, vault depth, and wear patterns on the glenoid in more detail. An additional benefit of obtaining an MRI is that fatty infiltration of the muscle belly and tears of the rotator cuff tendons can be identified.

View Large Image Download to PowerPoint

Figure 1. Axillary lateral radiograph to evaluate the glenoid.

View Large Image Download to PowerPoint

Figure 2. (A) Axillary MRI and (B) CT images to evaluate glenoid morphology.

Glenoid Morphology 

For descriptive purposes and preoperative planning, we find the staging system described by Walch and coworkers7 to be useful. The type A glenoid, which comprises greater than half of all patients (53.4%), demonstrates concentric wear. This can further be subdivided into type A1, which is minor wear (39.5%), and type A2 with glenoid cupping (14%). A type B glenoid (39.5%) has posterior wear, with type B1 having posterior wear (12%) and type B2 a biconcave glenoid (37.5%). Type C (5%) refers to glenoids with retroversion greater than 25° or underlying dysplasia (Fig. 3A and B).

View Large Image Download to PowerPoint

Figure 3. (A) Walch classification of glenoid wear patterns. (B) Walch type 2b glenoid (biconcave posterior wear). (Color version of figure is available online.)

Glenoid Resurfacing versus Humeral Head Replacement 

Initial evaluation of the patient involves taking into account the patient’s age, occupation, lifestyle expectations, and glenoid morphology. Multiple options exist for treatment of the glenoid, including no treatment, bone graft with reaming, reaming alone, and biologic resurfacing using a meniscal or Achilles allograft (Fig. 4A and B). Implantation with a glenoid prosthesis has demonstrated superior pain relief and function but may not be the right choice in a patient who is young and active, previously infected, or without adequate glenoid bone to support an implant.

View Large Image Download to PowerPoint

Figure 4. (A) Meniscal allograft fashioned into circle. (B) Meniscal allograft in vivo. (Color version of figure is available online.)

Prevention of Glenoid Loosening 

Treat the Glenoid Deficiency 

Recognizing and adequately treating glenoid bone erosion is necessary to successfully implant a glenoid prosthesis. The most common deformity is posterior wear, which can be addressed by several techniques. Most importantly, it is never recommended to use cement to “build-up” the posterior glenoid surface to support implantation. Noncontained cement has the tendency to fragment over time, leading to toggling and rocking of the unsupported glenoid. Based on preoperative assessment of glenoid vault volume, patients with an adequate vault should be treated with eccentric reaming of the anterior or “high” side of the glenoid to get a congruent surface and correct retroversion (Fig. 5A and B). In rare cases with severe loss of glenoid volume, the “low” side of the glenoid can be bone grafted using an allograft (Fig. 6A and B). A dual-radius designed glenoid prosthesis, which has a smaller diameter on the glenoid side and a larger diameter on the humeral side, can be used to match a larger humeral head to a smaller glenoid (Fig. 7). This prosthesis is useful in cases in which there is excessive posterior wear and bone loss. Also in patients in whom it is difficult to obtain glenoid exposure secondary to contracture, it is useful to use a small reamer and glenoid surface and still accommodate a large head. In a series of 16 patients treated at our institution with preoperative posterior subluxation, techniques used to correct the deformity included anterior capsulectomy (16 patients), reaming of the anterior glenoid (16 patients), dual-radius glenoid (8 patients), decreasing the humeral version (8 patients), and posterior plication suture (1 patient). With an average follow-up of 26 months, all 16 patients demonstrated correction of the posterior subluxation (Fig. 8A and B).

View Large Image Download to PowerPoint

Figure 5. (A) Eccentric anterior reaming to level face of glenoid. (B) Eccentric reaming can still leave adequate glenoid vault for implantation. (Color version of figure is available online.)

View Large Image Download to PowerPoint

Figure 6. (A) Severe wear may necessitate bone graft to build up the glenoid. (B) Postoperative X-ray after iliac crest bone graft implantation. (Color version of figure is available online.)

View Large Image Download to PowerPoint

Figure 7. Dual-radius glenoid trial implants displayed with standard glenoid trials implants and corresponding humeral head components. (Color version of figure is available online.)

View Large Image Download to PowerPoint

Figure 8. (A) Preoperative film of patient with hemiarthroplasty. The glenoid has posterior wear and the component is subluxed posteriorly. (B) After conversion to total shoulder arthroplasty, the posterior wear has been addressed by implantation of a dual-radius glenoid component.

Anterior glenoid bone loss is less common in patients with primary osteoarthritis, but may be found in patients with arthritis secondary to prior instability. If eccentric reaming is not an option, a bony procedure, such as a coracoid transfer (Latarjet) or iliac crest bone graft, may be performed. Adjustment of the humeral head retroversion can also correct anterior subluxation.

Gross glenoid bone deficiency is frequently encountered in the revision setting and should be addressed before reimplantation. While preoperative planning is helpful, most decisions will be made intraoperatively on inspection of the remaining glenoid bone. If bone stock is adequate, a primary reimplantation can be performed (Fig. 9A and B). If the defect is contained then impaction grafting to augment glenoid implantation is often successful. However, if there is any doubt, it is preferable to perform a staged procedure. If the patient has persistent pain, the second glenoid component can be implanted at a later date.4

View Large Image Download to PowerPoint

Figure 9. (A) Glenoid component loosening with severe bone loss. (B) Postoperatively, the glenoid component has been revised using impaction bone grafting.

Soft Tissue Considerations 

In addition, an intact rotator cuff is essential for a glenoid component. Rotator cuff deficiency in the shoulder leads to superior edge loading of the component. This abnormal force on the component results in toggling or a “rocking horse” glenoid and is associated with glenoid loosening and failure (Fig. 10).

View Large Image Download to PowerPoint

Figure 10. Glenoid loosening associated with superior humeral head migration and the “rocking horse” phenomenon.

Any excessive instability and humeral head subluxation can also lead to increased edge wear of the polyethylene. Correction of preoperative subluxation and appropriate component alignment are necessary to prevent abnormal force transmission to the prosthesis.

Glenoid Fixation 

A variety of fixation options for the glenoid component have been used in the past. Metal-backed components, with or without screw fixation, were found to have an increased association with failure, accounting for over 50% of failures at our institution before 19955 (Fig. 11). Compared with cemented all-polyethylene components, the survival rate of metal-backed components is inferior. A study by Boileau and coworkers2 confirmed this finding and found an absence of ingrowth and accelerated polyethylene wear with the metal-backed glenoid, leading them to abandon this implant completely.

View Large Image Download to PowerPoint

Figure 11. Glenoid implant loosening associated with a metal-backed implant design.

Cemented all-polyethylene glenoid components have demonstrated better results. The incidence of lucent lines is still high with this component, but progression of these lines is rare. Keeled designs have demonstrated significantly greater postoperative lucent lines (39%) compared with pegged designs (5%).8 A meta-analysis of shoulder arthroplasty presented at the American Academy of Orthopaedic Surgery annual meeting in 2005 looked at 1489 patients. Of the patients treated with a total shoulder arthroplasty, 7.7% required revision surgery. Revision was secondary to all-polyethylene glenoid loosening in only 1.7% of those evaluated. The need for glenoid revision after total shoulder arthroplasty secondary to glenoid loosening was less common than the need for glenoid resurfacing after an unsuccessful hemiarthroplasty.9

New technology has led to the development of Trabecular Metal (Zimmer, Warsaw, Indiana) implants, which provide an ingrowth surface with similar properties to cancellous bone. New designs of a glenoid component might make this an option for the future.

Conclusion 

In summary, glenoid loosening is an uncommon problem that is best treated with prevention. Careful patient selection and preoperative planning will help to define the osteology of the glenoid for primary implantation. Correction of both bone and soft tissue deformity is essential. Cemented all-polyethylene components are the best currently available technique for glenoid longevity. In the revision setting, a staged procedure may be necessary for adequate fixation.