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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Uncemented metal backed glenoids - high revision rate

Effect of glenoid cementation on total shoulder arthroplasty for degenerative arthritis of the shoulder: a review of the New Zealand National Joint Registry.

The first sentence of this article is worthy of our attention, "Despite the lack of literature showing improved results compared with cemented designs, uncemented glenoid components are still commonly used in total shoulder arthroplasty".

These authors used the New Zealand National Joint Registry data to compare the outcomes of uncemented and cemented glenoids in total shoulder arthroplasty for degenerative arthritis.

They retrieved data on 1596 patients, with a mean follow-up 3.5 years (range 2-10.7 years). 1065 had a cemented glenoid and 531 an uncemented component. The median follow-up time was 3.5 years (range, 2-10.7 years).There were no significant differences in any preoperative factors between the patients receiving cemented and uncemented glenoids. The revision rate for uncemented glenoids was 4.4 times higher than for cemented glenoids (1.92 vs 0.44 revisions per 100 component-years, P < .001).

Age <55 years was an independent risk factor for revision (P < .001). 

The uncemented prosthesis used in most of these cases has now been taken off the market. All of the cementless revisions were in shoulders receiving this component.

The most common reasons for revision are shown below.

These data are very interesting, in that (1) polyethylene failure and liner dissociation was the most common reason for revision in the uncemented metal backed glenoids, (2) rotator cuff failure was a common mode of failure, and (3) a high percentage of failures were related to posterior instability. It would be of great interest to know the glenoid pathology in these cases of posterior instability. 

The glenoid component remains the weakest link in total shoulder arthroplasty.

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Reverse total shoulder - baseplate fixation

Reverse shoulder arthroplasty glenoid fixation: is there a benefit in using four instead of two screws?

The authors used a cadaver model of reverse total shoulder to compare the baseplate mciromotion after fixation with two or four screws. In their model they found no significant difference.

This result may be relevant to implants that depend on osseous integration. However, our preference is for a system that provides immediate secure fixation with a central glenoid screw rather than one that requires bony ingrowth over time for glenoid component fixation.

This article is important, however, in that it indicates that with the Aequalis reversed glenoid baseplate, the micromotion increased with increasing cycles of loading and with increased applied loads. This brings up the question of how long one should wait after implantation of this system before loads are applied and what the maximal allowed loads should be at different times after surgery. Another important finding of this study is that greater deformation was measured during the anterior-to-posterior loading than during the superiorly directed loading. The authors suggest that rehabilitation protocols should limit activities that cause increased anterior–posterior loading after reverse total shoulder arthroplasty.

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Reverse total shoulder - glenosphere fixation - scapular notching

The impact of scapular notching on reverse shoulder glenoid fixation.

These authors used a plastic composite scapula model to evaluate reverse shoulder glenoid baseplate fixation and the effect of scapular notching. They used acyclic test to simulate 55° of humeral abduction in the scapular plane as a 750-N axial load was applied to induce a variable shear and compressive load. Before and after cyclic loading, a displacement test was conducted to measure glenoid baseplate displacement.

They found that scapulae without a scapular notch, glenoid baseplate displacement did not exceed 150-μm - a suggested threshold for osseous integration. For the scapulae with a scapular notch, glenoid baseplate displacement exceeded 150 μm in 2 of the 7 samples before cyclic loading and in 3 of the 7 samples after cyclic loading. The average pre-cyclic glenoid baseplate displacement in the direction of the shear load was significantly greater in scapulae with a scapular notch than those without a scapular notch both before and after cyclic loading.

We've previously posted on a related study: Radiographic analysis of the effects of glenosphere position on scapular notching following reverse total shoulder arthroplasty

Comment: Glenoid loosening rates for reverse total shoulders have been reported to range between 0% and 12%. This complication is multifactorial, depending on component design, glenosphere placement, surgical technique, the degree of glenoid bone reaming, bone quality and the loads applied by the patient. Some designs, such as the one studied here, depend on bone ingrowth for fixation - thus the attention to the described 'threshold for osseous integration'. The problem with bone ingrowth is that one cannot be sure how long it takes before good fixation is achieved and, in each case, how long the shoulder should be protected from loading to assure that the micromotion threshold is not exceeded. Other designs, such as the one shown here provide immediate fixation without need to wait for bone ingrowth. The clinical environment surrounding each patient's reverse total shoulder is unique and this variability cannot be simulated in the lab. Only long term clinical studies will reveal what designs and techniques are best suited for which patients.

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Fixation of the glenoid component - preserve cortical bone!

Glenoid implant orientation and cement failure in total shoulder arthroplasty: a finite element analysis.

The authors propose that the risk glenoid implant loosening is minimized by correction to neutral version, complete implant-bone contact, and bone stock preservation. While the second and third of these make good sense, the value of correction of glenoid version to neutral has yet to be proven in robust clinical studies. 

The authors created computer models for homogeneous cortical bone and for heterogeneous cortical-trabecular bone model. They predicted the risk of cement mantle fracture when a 750-N load was applied.

Basically the question here is how to manage a retroverted glenoid. Does one place the glenoid component on the cortical bone without attempting to 'correct' the version:

Or does one 'correct' the version by reaming the anterior bone into the cancellous bone:

In the heterogeneous bone model (which most closely represents what we see clinically), complete correction resulted in the highest risk of failure. A positive correlation was found between the risk of cement failure and amount of exposed trabecular bone.

So the lesson from this model seems to be 'preserve cortical bone'.

There may be another lesson as well. Since the model measured failure by the stress in the cement mantle beneath the face of the component (shown in red above), it seems preferable to avoid cement between the plastic and the bone and, instead, achieve bone-prosthesis contact by careful reaming to fit the back of the component. Our method is shown here.

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Reverse total shoulder glenoid fixation

Initial glenoid fixation using two different reverse shoulder designs with an equivalent center of rotation in a low-density and high-density bone substitute

Four of the authors of this paper are employed by Exactech Inc. and a fifth receives royalties from the company that makes the reverse total shoulder design associated with better results in this study.

This study sets out to evaluate initial reverse shoulder glenoid baseplate fixation in 2 reverse shoulder designs having an equivalent center of rotation in low-density and high-density bone substitute materials. There were many differences between the baseplates of the two systems tested (glenoid plate profile, contour of back of baseplate, number of screws, shape of peg, etc).

 The Zimmer design (circularporous reverse shoulder) was associated with approximately twice the micromotion per equivalent test than the Exactech design (oblong-grit-blasted reverse shoulder). Additionally, 6 of the 7 circular-porous reverse shoulders failed catastrophically in the low-density bone model. None of the oblong-gritblasted designs failed in the low-or high-density bone models and none of the circular-porous designs failed in the high-density bone models after 10,000 cycles of loading.

Comment: Perhaps the most important finding in this study is that bone quality is a factor in fixation of the glenoid component in reverse total shoulder leading us to the conclusion that reaming of the precious glenoid bone stock needs to be minimized to preserve as much of the quality bone as possible.  A second observation is that both of the designs tested here depend on bone ingrowth for a substantial part of the fixation - however, bone ingrowth could not be tested in this model system; instead, the study focused on initial glenoid fixation as indicated by 'micromotion' and 'catastrophic failure'.

We have no fiscal interest in an implant system. The system we prefer (shown here) does not depend on ingrowth (which under the best of circumstances takes some time), but rather provides excellent initial fixation from compression of the base plate against the prepared glenoid bone - especially if the glenoid subchondral plate is preserved.

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Reverse total shoulder - baseplate fixation

Reverse shoulder arthroplasty glenoid fixation: is there a benefit in using four instead of two screws?

The stated purpose of this study was to determine whether a 4-screw construct provides more baseplate stability than a 2-screw construct.

A flat-backed glenoid baseplate with 4 screw hole options was implanted into 6 matched pairs of cadaver scapulas using standard surgical technique. Within each pair, 2 screws or 4 screws were implanted in a randomized fashion. A glenosphere was attached allowing cyclic loading in an inferior-to-superior direction and in an anterior-to-posterior direction. Baseplate motion was measured using 4 linear voltage displacement transducers evenly spaced around the glenosphere.

There was a difference: at the 30% loading level, with 2-screw fixation, the average central displacement was 0.17 mm with superior loading, 0.21 mm with anterior loading, and 0.22 mm with posterior loading. With 4-screw fixation, the average displacement was 0.12 with superior loading, 0.15 with anterior loading, and 0.14 with posterior loading, but these differences were not significant.

The authors conclude that using fewer screws could potentially lead to a reduction in operative time, cost, and risk, with no significant negative effect on overall implant baseplate motion.

While these findings are of interest, one must be cautious about applying them clinically, in that the quality of bone in these cadaver shoulders may be better than what is found in the typical patient having a reverse total shoulder. Furthermore there may be differences among designs with respect to the security offered by the central screw or peg as well as the use of locking vs. non-locking screws. 

It is of greater interest that there was a significant difference in the central displacement with increasing number of loading cycles and a significant difference in the central displacement with increases in loading magnitude. This cautions us to counsel our patients about loading a reverse total shoulder - it may not be forgiving.

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Glenoid fixation in reverse total shoulder

Improving glenoid-side load sharing in a virtual reverse shoulder arthroplasty model.

This article again shows the relentless dedication of the senior author to improving the art of reverse total shoulder.

The topic of glenoid base plate fixation is of interest in that in the reverse total shoulder much greater loads are applied to this fixation that with anatomic total shoulders. 

In a prior article we have suggested that optimizing contact between the baseplate and the bone enhances resistance to component loosening. The topic of fixation has also been addressed in a previous post.

In this article the authors examined the hypothesis that increases in implant-bone contact improves stability through load sharing with respect to baseplate fixation. This hypothesis was tested in 3-dimensional models in which total implant-bone contact area was compared for two conditions: (1) baseplate flush with bone and no additional glenosphere contact, or (2) baseplate and glenosphere undersurface in contact with bone.  Micromotion and stress were computed for each size of implant in the two conditions. They found that increased total contact area when the glenosphere is in contact with bone reduced stress and micromotion.

Achieving glenosphere contact with bone can be achieved in at least three ways (1) designing the implant so that the glenosphere is flush with the baseplate, (2) using bone graft to make up the gap between the medial aspect of the glenosphere and the bone and (3) and reaming the glenoid so that the baseplate is inset, allowing the glenosphere to contact the bone.

As is pointed out in this article, each of these approaches carries the risk of incomplete seating of the glenosphere on the baseplate giving rise to the risk of dissociation of the glenosphere. Assuring complete seating of the glenosphere is a key part of this surgery. We surely don't want the Anne Boleyn syndrome where the head is separated from the body!

There is another risk with the third approach listed above - reaming through the thin subchondral bone of the glenoid typical of folks needing a reverse total shoulder. Reaming through this thin layer of bone places the baseplate on softer cancellous bone, making failure more likely. Our practice is to minimize reaming, never using power, just to the point where the baseplate will be completely supported. Furthermore, we do not seek to angle the glenoid prosthesis inferiorly in that this also requires compromising the thin subchondral bone as shown in a gentleman from Los Angeles for whom we did a reverse last week.

In the post operative film, note that the subchondral plate is still present.

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Accuracy of obtaining optimal base plate declination in reverse shoulder arthroplasty JSES

Accuracy of obtaining optimal base plate declination in reverse shoulder arthroplasty JSES

In this paper the authors describe a method for measuring the inclination/declination of the glenoid in relation to a reference line from the superior medial angle of the scapula to the superior margin of the glenoid. Unfortunately this reference line is not accessible in the OR. We note that there are a number of different references for measuring glenoid inclination and that the reference framework for the 'conventional wisdom' that the ideal position for the baseplate include 15 degrees of declination is not clear. As the authors point out, the reference cannot be the inconsistent orientation of the face of the preoperative glenoid, which in their study varied from -27 to + 28 degrees. Even in their experienced hands, the postoperative inclination of the baseplate varied widely from -23 to 44 degrees.

In our approach, we are less concerned about declination and more concerned about the superior/inferior position of the glenoid, the freedom from contact between the adducted humeral component and the scapula, and preserving quality bone on which the baseplate can be placed, recognizing that in prior studies, we have shown that the quality of bone on which the base plate is placed may be a critical determinant of the strength of fixation. Thus one must be cautious about reaming away bone of good quality to achieve a desired declination.

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