Anatomic total shoulder replacement, Lima SMR stemless humerus with TT glenoid
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Anatomical total shoulder replacements are in widespread use for the treatment of patients with arthritic shoulders. Over many years the designs have changed in light of outcomes and research, particularly related to failure mechanisms. An intact rotator cuff insertion is now recognised as essential for optimal outcome from an anatomical configuration shoulder replacement. This is because one of the functions of the rotator cuff is to depress the humeral head and centre it in the glenoid during shoulder movement, particularly elevation. In the absence the superior rotator cuff tendon supraspinatus, the humeral head rides up and migrates proximally such that it may articulate with the under surface of the acromion. This will mean there is point loading on the superior edge of a prosthetic glenoid component resulting in excessive eccentric wear and the ‘rocking horse effect’ resulting in loosening of the component. While analgesia may be adequate, function is likely to be poor and risk of glenoid component wear and loosening and hence failure is high.
Many Orthopaedic companies have developed such shoulder replacement implants. In this case, I am using the stemless humeral implant on the anatomic system from the Italian company Lima. The stemless humeral component is constructed on a 3D printer to create a single piece of trabecular titanium. Hip surgeons have used the same trabecular titanium for successful reconstruction of the hip acetabulum with impressive bony ingrowth of the implant. In the shoulder, if the patient’s proximal humeral metaphyseal bone is not too soft then impaction of the trabacular titanium core humeral implant gives excellent primary fixation and promotes future bone ingrowth securing the platform component.
This is a bone preserving procedure avoiding the need to use an intramedullary stem. The same humeral core component is used for reverse total shoulder replacement as also described in Orthoracle and this facilitates revision of an anatomic replacement to reverse. The original Lima anatomic shoulder replacement uses a metalback glenoid component, fixed with press-fit and cancellous screws, into which the polyethylene component is inserted. It is very straightforward to perform revision surgery to switch the articulating components leaving the glenoid baseplate and humeral core implants, which should be solidly fixed to bone, in place.
There have been concerns with metal glenoid baseplates concerning eccentric wear of the polyethylene resulting in the humeral head component articulating with the metal rim of the baseplate. Dissociation of the polyethylene liner from the metal baseplate has also been recognised. To reduce the risk of such problems Lima have developed the TT hybrid glenoid component. The majority of the implant is polyethylene with a central peg attached to a trabecular titanium (TT) core and two much smaller peripheral pegs to ensure rotational stability which can be fixed with a tiny volume of cement, hence the hybrid label. The central TT peg is impacted into the prepared glenoid and will encourage bone ingrowth just as the humeral core implant.
If in the future revision surgery from anatomic to reverse is required, then the system has been designed to allow the polyethylene to be drilled and dissociated from the central TT core and discarded. The central peg core is firmly embedded in the glenoid into which a male to female metal baseplate can be inserted onto which the glenosphere is fixed for the reverse geometry shoulder replacement.
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Author:Mr Mark Crowther FRCS (Tr & Orth)
Institution: The Avon Orthopaedic Centre, Southmead Hospital, Bristol, UK.