Modified Slocum’s DARthroplasty
Description of the modified Slocum’s shelf augmentation with figures, images and video support.
The technique that we will describe was developed after working several years with Slocum’s extra-capsular Shelf technique – the DARthroplasty. It is our opinion that this technique is specially valuable in juvenile dogs with severe coxofemoral subluxations and/or severe underdevelopment of the acetabulum. The acetabular augmentation is adapted to each individual case according to the dysplastic morphology and dynamic situation of the joint. The main goal should be to obtain a sufficient and robust coverage of the femoral head and not worry too much with excessive coverage, which clinical consequences will not be important. Nevertheless, we should strive not to overcorrect the deficient coverage.
Regarding indications, we think the acetabular augmentation does not have a well defined age limit. In our point of view, the ideal candidate is between 4 and 6 months of age due to the inherent biological plasticity and lesser degree of secondary changes to the joint. The 7 to 9 month old age group will have a favorable prognosis depending on disease status. We’ve had several cases with good results in the post 9 month old group. But less biologic plasticity and more secondary changes to the joint, which is frequent in this group, demands a more rigorous case selection. We emphasize that the patients we are referring to suffer the most severe forms of dysplasia. We are not concerned with the compensated, mild, symptom-free spectrum of the disease.
The technique is no longer indicated when the femoral head has overridden the dorsal acetabular rim, forming a false acetabulum over the rim. In general, joints with far advanced degenerative joint disease will have a reserved prognosis with this conservative hip surgery. Namely, the severe loss of articular cartilage (exposing the subchondral bone) and the severe deformation of the femoral head with exuberant osteophytes. We should be cautious to consider the DARthroplasty in these situations.
1 – Access to the joint capsule and dissection
We execute a caudal access to the coxofemoral joint. The cutaneous incision is parallel to the cranial limit of the biceps femoris muscle, from the sacrotuberous ligament to the third femoral trochanter. The biceps is retracted caudally and the superficial and middle gluteal muscles are retracted cranially. The sacrotuberous ligament is identified and severed (to avoid the entrapment of the sciatic nerve between this ligament and the expanding bone graft – Slocum).
We deepen the dissection to the level of the joint capsule where a branch of the caudal gluteal artery runs over the gemelli muscles (Slocum). The artery is coagulated with bipolar cautery or ligated. All muscle fibers are dissected free from the dorsal capsule. Next, we perform subperiosteal elevation of the muscles from the dorsal acetabulum in the extent needed for the bone groove that will be created (“d” fig 1). We must be careful not to severe capsule insertion fibers on the dorsal acetabulum. Frequently it is possible to create a “pocket” between the articularis coxae muscle and the capsule using blunt dissection with a curved fine pointed instrument. Latter, this “pocket” will receive the cranial end of the first bone strip assuring a good ventrodorsal level to the augmentation (fig. 2). Caudally, the gemelli muscles and the internal obturator tendon are dissected free from the capsule, creating a caudal pocket to insert the caudal ends of several bone strips. During this part of the procedure it is useful to put the hip in abduction and gain some more available space to work.
a – Internal obturator
b – Deep gluteus
c – Capsule
d – Groove
e – Articularis coxae
f – First strip of the first layer
2 – Gaining a stable space for the technique and creating the bone groove
One strategy to create a stable comfortable space to work is to insert a 2 or 2,5 mm Steinmann pin in the dorsal acetabulum on the most cranial area of the groove that will be created (“d”, fig.1). If after it’s insertion we bend it cranially this curved retraction pin will hold the muscles (gluteals and piriformis) and will not interfere with the rest of the technique. Alternativelly to the retraction pin, an assistant can hold an appropriate tissue retractor. In the joints with severe effusion, joint fluid aspiration will permit to gain more working space. Abducting the coxofemoral joint will also increase space (ex. with radiographic positioners or other means).
The groove is created removing a strip of cortical bone from the dorsal acetabulum (from it’s cranial to it’s caudal limit and to the level of bleeding spongiosa) with a 4 mm Lexer gouge or other means. The groove is positioned just dorsal to the dorsal limit of the capsular insertion (“d”, fig.1).
3 – Bone graft harvest
The bone graft is harvested from the ipsilateral iliac wing. The skin incision starts half way between the sacral and coxae tuberosities at the cranial margin of the wing and follows the direction of the great trochanter (Slocum). The length of the incision is the required one for the safe and effective removal of the bone strips. A typical value would be 10 cm for a 35 Kg Dog. The access is continued to the deep fascia which we cut on the same direction of the skin incision, avoiding damage to muscular fibers (see photo). A periosteal elevator is placed against the bone on the cranial margin of the iliac wing and advanced caudally to the caudal end of the harvest zone. We execute leverage action with the elevator to split the muscle fibers while keeping it’s tip in contact with the bone(Slocum) (watch video).
In the next step, the entire wing is exposed. To access the most ventral and dorsal areas of the cranial wing we execute 2 incisions (one ventral and the other dorsal) on the muscular insertions at the iliac crest, extending the first fascial incision to a “T” shape.
The first strip is harvested along the central axis of the iliac wing and body with a curved 10 mm Lexer gouge. The following 2 strips are harvested parallel to the first, one dorsal and one ventral. All (or almost all) lateral cortex is removed in this fashion. Depending on the thickness of both the individual bone and of the first strips harvested, more spongiosa strips can be removed deeper in the iliac wing. In dogs older than 7 months it may be necessary to thin out the cortical layer of bone so that the harvest is easy and precise. The thinning is done with a burr or other means, avoiding high temperatures.
Incision in the gluteal fascia
Corticocancellous and spongiosa strips harvested from the same iliac wing
4 – Graft placing
The first strip to place over the femoral head is the most lateral one. We choose for this position the strip with the best conformation. Ideally it is a spongiosa strip. In the 4-6 month old dogs this is not so important due to the porosity and malleability of cortical bone. We insert the cranial end of the strip in the “pocket” underneath the articularis coxae (Fig.2). Next, we manipulate the tendon of the internal obturator to ease the insertion of the caudal end of the strip in the caudal “pocket”. These 2 “pockets” stabilize position and ventrodorsal level of the augmentation so that the graft is loaded, which is essential for its incorporation and maturation. The second strip is placed medial and parallel to the first. The next strips are placed in the same fashion until the created groove is covered (Fig.3). Spongiosa is the preferred bone for this first layer. We create a second layer on top of the first (Fig.4). The sensation is that we are filling a tight space with a large bone mass. To ease insertion in the cranial “pocket” we abduct the hip. To ease insertion in the caudal pocket we abduct and externally rotate the hip if necessary.
It is not expected that the strips are similar between them. We use them to construct an augmentation that is as compact as possible (avoiding dead space) and with a robust thickness (ventrodorsal dimension). We can fill in the dead spaces inside the augmentation with fragments of spongiosa that are obtained in the iliac wing after the strips are harvested (for example, with a Volkmann spoon). For better compaction and cohesion of the augmentation we can compress (manually or with an instrument) the second layer against the first, molding the bone mass.
In this modification of the original Slocum technique no sutures are used to stabilize the graft. The graft is stabilized by the 2 pockets and by the restriction of space. The space created under the muscles is just the sufficient to accommodate the graft.
First layer of strips
g – Second layer of strips
5 – Closure
In both accesses, the first layer to be sutured is the deep fascia. We execute the usual plane by plane closure.
6 – Postoperative period
The owner is asked to restrict intense propulsive activities. That means not allowing jumping and running until full integration and maturation of the graft. This usually takes 3 to 4 months.
Acknowledgement: the images that support the text were created by Hardfolio (www.hardfolio.com)