Porous Metal Glenoid Augment PMGA

Published data has indicated that approximately 30% of arthritis shoulders will have glenoid retroversion that should be corrected for maximum longevity of an implanted glenoid component as part of an anatomical total shoulder arthroplasty.

Options to correct such retroversion include eccentric reaming and bone grafting. Several augmented glenoid components are available, but the results have been variably documented, and outcomes have been relatively poor.

The shift to increased use of the reverse replacement has been in part fuelled by the lack of a satisfactory solution to address glenoid retroversion for cuff intact osteoarthritis. Bone grafting is an accepted option, but it is technically difficult and has a high graft resorption and, thus, failure rate.

Porous metal, designed as an ingrowth deformity correction device, has a successful and long history in the lower limb and has now been adapted to a similar role in the shoulder.

A long-term prospective study has confirmed the efficacy of the porous metal wedge.

Clinical Series

Follow-up radiographs

Technique Instruction for PMGA (Porous Metal Glenoid Augment)

Overview:

These implants are specifically designed to address glenoid retroversion where the wear pattern generally conforms to the Walch grade B2 and C. The PMGA implant is designed to match several different vendor glenoid components of Total Shoulder Arthroplasty systems and is regulated and covered by the Private Practice funding list. Contact Signature Orthopaedics for details.

The use of this augment is analogous to the surgical approach of using a bone graft to address posterior glenoid bone loss, into which an HDPE glenoid component is cemented. Specific surgical management can vary, consistent with a surgeon’s experience and the patient's anatomy.

Pre-operative planning:

An adequate appreciation of the glenoid morphology is essential as part of surgical planning. Characterisation of glenoid deformity is best done using CT scanning, ideally with 3D reformation. While not essential, a 3D virtual surgical planning capability will facilitate optimal augment selection and positioning and provide an appreciation of specific anatomical features that may present challenges during the arthroplasty. The anterior scapula is a critical and generally consistent anatomical reference point for assessing the glenoid version; thus, ensuring this region has anticipated morphology will confirm the appropriateness of using it as an alignment reference.

Implant TMGA (Trabecular Metal Glenoid Augment) Range:

            30 degree large
           15 degree large
           30 degree medium
           15 degree medium

Special instruments (supplied as TMGA Kit):

Anterior scapula glenoid alignment guide
Trial implants:
30 degree - large and small
15 degree - large and small
TMGA peg hole stabilizer
Surgical bur – 8mm x 20 mm/tree profile / double cut bur
Bone screws (3.5mm non-locking, low profile head)
Transparent glenoid surface checker (not yet available)

Specific Additional Equipment (supplied as part of usual surgical set-up)

Curved Kocher’s forceps – to stabilise TMGA onto the glenoid during initial drilling
2.5mm drill and depth gauge – to drill through TMGA to facilitate screw fixation.

Standard Shoulder Arthroplasty Instruments

Usual instruments as supplied by the vendor to facilitate glenoid exposure, placement, and cementation.
Fast-setting PMMA cement with an antibiotic.

Surgical Technique

The standard approach will be abbreviated, and specific TMGA steps will be detailed.

Patient positioning, anti-infection and thromboembolic prophylaxis as desired
Routine skin preparation and draping. Alcoholic Chlorhexidine is currently recommended.
Standard skin incision and development of deltopectoral interval.
Biceps tenotomy and tenodesis, plus release rotator interval along line of biceps to base of coracoid process.
Subscapularis tendon release by either osteotomy (typically in men) or sharp dissection (typically in women). Tag and reflect.
Dislocate the humeral head and trim osteophytes to define the margin.
Excise the head using the standard instrument-specific approach. Beware of “flat cap deformity”.
*** Excessive posterior head resection may compromise posterior rotator cuff***
Prepare humeral canal and insert trial stem.
Mobilise humerus to display glenoid. Locate and protect the axillary nerve.
Resect anterior capsule and insert anterior glenoid retractor.
Insert the posterior humeral retractor and progressively excise the anterior, inferior and posterior capsules.
Survey glenoid to assess version. Insert an Anterior scapula glenoid alignment guide with relevant trial implants to assess the correction needed.

14. When adequate glenoid exposure is confirmed, use the provided bur to contour glenoid face to match back side of augment, with modest version correction as needed.
*** The TMGA is available in 2 different correction angles (15 degrees and 30 degrees), and 2 sizes. The large implant is suited to the 46mm and 52mm glenoid, and the medium to the 40mm glenoid component. To achieve the desired glenoid version of less than 10 degrees of retroversion, use the following correction algorithm, with a small or large implant depending on the inferior to superior dimensions of the glenoid. The maximum acceptable correction by eccentric reaming is 10 degrees. ***

Retroversion                Amount of glenoid preparation.                                    Implant used
10°                             eccentric ream to neutral                                  standard implant, no PMGA
15°                             minimal ream to smooth                                   15° PMGA plus standard glenoid
20°                             eccentric ream to 150 and smooth                    15° PMGA plus standard glenoid
25°                             increase retroversion to 300                              30° PMGA plus standard glenoid
30°                             minimal ream to smooth                                   30° PMGA plus standard glenoid
30+°                           eccentric ream to 30° and smooth                     30° PMGA plus standard glenoid

Bone graft can be added under the PMGA to address excessive retroversion or glenoid defects as may occur in revision surgery.

15. Check final preparation correction using a trial implant and Anterior scapula glenoid alignment guide.

16. Open the definitive PMGA implant and manually position it in the desired location on the glenoid face. The TMGA should be centrally positioned, 3-4mm posterior to the anterior glenoid margin, and should be positioned “below the horizon” of the anterior rim. The standard glenoid component is longer than the TMGA on its anterior margin, which allows the glenoid component to rest on the native glenoid anterior rim and thus avoids both excessive anterior positioning of the glenoid component and overstuffing the joint

17. Stabilise the TMGA in the desired position using a Kocher’s forceps between the lower peg hole and the anterior wall of the scapula.

18. Once reliably held in position, drill the upper anterior drill hole and insert the screw. Depending on access and available drill holes, drill the lower posterior drill hole and insert the screw. Reposition Kocher’s forceps if necessary, and drill the lower anterior drill hole. Two or three screws should be sufficient.

19. Ensure Kocher’s forceps are stabilising TMGA, and using a large peg drill from the arthroplasty set, drill through the TMGA to prepare a central peg hole into the glenoid bone.
*** Ensure drilling is perfectly in line with the TMGA peg hole to avoid the drill jamming and dislodging the augment. Use the arthroplasty drill guide to assist if concerned****

20. Insert TMGA Peg hole stabilizer into central TMGA peg hole and either reposition or remove Kocher’s forceps as access and augment stability dictates. Drill through upper and lower TMGA peg holes into glenoid. Trim anterior rim to ensure glenoid bed (anterior rim and TMGA) is flat.

21. Insert trial glenoid component to ensure satisfactory seating. Use an arthroplasty drill guide to enlarge and line up the holes if the glenoid component does not fit easily.

22. Re-check glenoid version with TMGA and trial implant using Anterior scapula glenoid alignment guide.

23. When cementing the glenoid, multiple cement insertions and pressurisation are recommended. This is done by injecting cement into the peg holes and then inserting the pegged impactor from the arthroplasty set. This is typically done three times. However, only insert small amounts of cement deep into the actual glenoid drill hole to avoid cement escaping into the TMGA-glenoid interface and potentially compromising bony integration into the augment.

24. Apply a small amount of cement to the back side of the glenoid component and implant.

25. Remove excess cement before it sets while stabilising the glenoid component.

26. Once the cement has hardened, VERY carefully deliver the humeral head into view.
***Due to the degree of posterior positioning and translation of the humerus and the long posterior face of the TMGA and glenoid component, premature inadvertent external rotation or anterior traction on the humerus may dislodge the glenoid construct. Use lateral traction with a hook and careful manipulation to disengage the humerus. Commencing the anterior mobilisation of the proximal humerus prior to removing the posterior retractor can assist in disengagement and reduction. ***

27. Once the humerus has been presented, standard humeral stem insertion and head trialling are performed. Because the glenoid is now more anterior and lateral than pre-operatively, the head height is generally less than would be anticipated if using the excised humeral head as a reference.

28. Once the definitive humeral stem and head are inserted, reattach the subscapularis tendon with a non-absorbable suture as desired. Formal closure of the rotator interval will significantly enhance the subscapularis tendon repair strength and reduce the tendency for superior humeral head migration. Access for such closure is improved by extending the shoulder and gently retracting the deltoid.

29. Further wound closure as desired.

Post-operative management:

Typically in a sling of collar and cuff for 4 weeks with limited exercises, including pendular and gentle flexion to horizontal, plus intermittent light functional use out of the sling at the waist and lower chest level.

At 4 weeks, progress to normal activities with specific attention on rotator cuff strengthening. Avoid excessive load or stress for 3 months.

Post-operative Radiological surveillance:

Post-operative Day 1 – AP glenoid, AP and lateral view of shoulder.
Post-operative 4 weeks - AP glenoid, AP and lateral, plus axillary view of shoulder
Post-operative 6 months, 12 months and then yearly - - AP glenoid, AP and lateral, plus axillary views

Patient outcome assessment:

ASES / Oxford Shoulder score / VAS pain score / forward elevation measurement -
- Pre-op / 3 months / 6 months / 12 months / yearly thereafter.

Additional Instruction

** Note – The optimum or most recent technique may vary from that shown in this guide as the instruments are progressively refined and improved. Prior to performing this technique, review the latest instruments and ensure you are familiar with the optimum surgical technique and the individual components and application — including indications, contraindications, warnings, cautions, and instructions.

PMGA supplied by: