Sunday, May 31, 2015

Shoulder joint replacement - fixation of the head component to the humeral bone

Fixation of the head component to the humeral bone
The methods by which the humeral head component is fixed to the humeral bone range from stemless and mini-stemmed to long stemmed and include fixation with cement, press fit, tissue ingrowth and impaction grafting.

There are at least three issues to consider. (1) The shape of the humeral canal is curved longitudinally and is elliptical in cross section; some canals are cylindrical while others are funnel shaped. (2) Fixation needs to be achieved without risk of loosening on one hand and without risk of fracture on the other. Intramedullary reaming to achieve a diaphyseal press fit preferentially removes bone from the anterior and posterior endosteal cortex; this endosteal notching at the prosthesis tip creates a risk of periprosthetic fracture. (3) Any shoulder arthroplasty is at potential risk - as high as 10% - for revision surgery to manage infection, loosening, malposition, fracture or instability, so that it is important that the humeral component be completely removable without seriously damaging the humeral bone stock. 
We have found that these three issues are best addressed by a 120-140 mm long smooth (non-ingrowth) humeral stem that fits in the canal without diaphyseal reaming and that is fixed with impaction autografting using bone harvested from the resected humeral head rather than with cement. Cancellous allograft is used in revision cases or where the amount of bone available for grafting is insufficient


Because it is difficult to achieve secure impaction with a cylindrical humeral body, such as that shown below
we prefer a stem with a proximal flare: the metaphyseal portion of the prosthetic body is thicker in the anteroposterior and medial-lateral dimensions than the diaphyseal portion - a configuration that enables secure fixation without driving the stem tightly into the humeral cortex distally.



The effectiveness of this technique has been documented by a laboratory study showing the increment in quality of fit and fill and by clinical follow-up studies. This method of conservative reaming and broaching combined with impaction grafting avoids the most common problem with press-fit humeral components: too high positioning of the prosthesis because of incarceration of the distal end of the stem in the humeral diaphysis leaving the prosthesis prominent with respect to the tuberosities and the glenoid as shown in the cases below.


Tip incarceration and incomplete seating of the humeral component may result from choosing a stem sized based on the AP x-ray, which usually shows a wider canal width than that on the axillary because of the oval shape of the humeral canal. Obviously, the problem of a too high and tight stem fit cannot be solved by trying to hammer the prosthesis down further, rather our solution is to change to a smaller implant stem diameter and fill the void by impaction grafting. We like to say that the impaction grafting approach allows the surgeon to ‘get it wrong but still get it right’. In a way this is similar to the method used by Procrustes, the legendary Greek innkeeper, who fit his ‘guest’ to his bed, rather than fitting the bed to the guest. If the prosthesis sits too far distal, the problem is solved by adding more autograft. If it sits too far proximal, a smaller stem can be used. If the position of the prosthesis is not ideal, it can be fine-tuned by selective placement of the graft. If an impaction grafted, uncemented, non-porous coated stem needs to be revised, it can usually be disimpacted without concern about cement removal and without the need to perform a humeral osteotomy as may be necessary to remove a prosthesis with ingrowth surfaces, trabecular metal, or platform (modular) stems. In special situations it may be necessary to saw off the tip of the prosthesis to the necessary length and fix it with cement 

We reserve the use of long stems for cases in which an area of cortical weakness needs to be bypassed as shown below.


We do not find systems with ‘platform’ stems attractive because many failures of anatomic arthroplasties are related to improper placement of the stem as shown below.

It seems doubtful that a platform design would facilitate revision of the cases shown below to a reverse [Fig 144 U2414709 too high biomet to rsa]. 



As long as the stem is not cemented and does not have a bony ingrowth surface, conversion to a reverse is usually straightforward


While some systems offer adjustable neck shaft angles, we have found this added complexity to be unnecessary in that a standard 45 degree cut can accommodate normal, varus and valgus anatomy.

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