The Management of Version Abnormalities and Angular Deformities in Developmental Dysplasia of the Hip

Abstract

Implantation of a femoral stem in a dysplastic femur poses many problems. Most common abnormalities are excessive anteversion and valgus angle of the femoral neck. This makes it impossible to use conventional stems. Suitable options are distally fixed stems with small metaphysis or proximally modular stems. Femurs with previous varus or valgus osteotomies show more severe deformities that sometimes require performing an osteotomy to obtain adequate correction. Again, distally fixed stems or proximally modular stems are suitable, the latter having the advantage that, by loading the femur proximally, they load the osteotomy therefore enhancing the healing.

It is established that the proximal femur has different degrees of deformity in developmental dysplasia of the hip (DDH). The most common abnormalities include excessive valgus and anteversion of the femoral neck. The development of these abnormalities implies a reduced contact area between the femoral head and the acetabulum, which is small in size and shallow in shape. A reduced contact area leads to increased stress on the cartilage, which causes it to wear, bringing about future arthritic changes.

In preprosthetic times, many surgeries were developed to improve the congruity between the head and the socket, both on the femoral and acetabular sides, to reduce wear of the cartilage and delay the onset of arthritic changes. These surgeries continue to be performed in some young patients. However, with the improvement of hip replacement surgery and the durability of new materials (eg, highly X-linked polyethylene, metal-on-metal, ceramic-on-ceramic), hip replacement is now performed in young patients. As a result, osteotomies have become rare. Because a large number of osteotomies have been performed in the past, surgeons must frequently implant hip prostheses in patients who had osteotomies for DDH that resulted in deformities. Management of femurs that have been operated on and femurs that have not been operated will be addressed in this article.

Options for Non-Operated Femurs

In femurs that have not been operated on, options for managing the anteverted femoral neck are available. The first option is to use a distally fixed implant with a narrow proximal part (such as the Wagner Conus) so that the deformed metaphysis can be ignored. This surgical technique is simple, and a relatively small inventory is needed. However, stress relief osteoporosis in the proximal femur will occur. Stress relief osteoporosis coupled with the difficult removal of the implant causes distal fixation to be a less desirable option than others, especially in younger patients who are more likely to have revision surgery during their lifetime.

Different types of modular stems provide other options for femurs that have not been operated on. In some types, the neck and the metaphysis are monolithic and are coupled with different distal parts. These stems cannot alleviate the complication of excessive anteversion because the placement of the metaphysis in the existing bone causes the prosthetic neck to replicate the pre-existing situation. If one of these stems is used, then a surgeon must perform a derotating osteotomy distal to the junction of the two components to reduce the anteversion and reconstruct a physiologic anato my. Other stems have the ability to independently place the metaphyseal component and the neck. This can be achieved with interchangeable necks, with different versions fixed to the metaphysis, with an oval reverse Morse taper, or with a round Morse taper that can be locked with a screw in different degrees of anteversion. It is unfortunate that both of these stems tend to be bulky and cannot be made small. If the stems were made small, then strength of the prosthesis would be compromised. In patients with DDH, the proximal femur is small and sometimes shows a reverse mismatch between the metaphysis and the diaphysis, with the metaphysis being smaller. For this reason, the ideal prosthesis for DDH must be small in the proximal part.

The last option for femurs that have not been operated on is a proximally modular prosthesis in which the neck and the distal part are one single piece and the metaphyseal component is locked on the stem with a Morse taper. The Morse taper can be impacted in the position to best fill the bone, allowing a surgeon to place the neck in the correct version. This system has many advantages. Because the load is transferred from the neck to the distal part, which is a monoblock, the metaphyseal sleeve is loaded only in compression and can be made thin to fit small femurs. With this type of prosthesis, an osteotomy is not needed to correct the anteversion unless a surgeon wants to reposition the greater trochanter (which might be too posterior) to the correct lateral position to regain a more physiological lever arm of the glutei muscles. Although most surgeons agree that osteotomies are sometimes necessary, debate exists about how often they may be needed. In the author’s experience, osteotomies performed to reposition the greater trochanter alone are rare.

The angular deformity that occurs in femurs that have not been operated on is excessive valgus. An option for managing excessive valgus is a thin metaphysis distally fixed stem. Stems with a physiological shape at the calcar are unsuitable because the large metaphysis prevents the stem from seating properly and results in distal undersizing. A stem with modular sleeves of different shapes is a good option because a surgeon can select the most suitable shape for the sleeve and place the triangle in the best position. (In some patients, the triangle can be inserted in the calcar anteriorly, or in the greater trochanter.)

Options for Operated Femurs

In patients with femurs that have been operated on the two most common situations are the valgus osteotomy and the varus osteotomy. The valgus osteotomy creates an excessive valgus proximal femur (Figure 1A). It is possible to perform the osteotomy again and place the proximal femur in the original position. However, this option is not usually chosen because it is not necessary and adds complexity to the procedure. The options chosen most often are straight stems with distal fixations and/or stems with modular sleeves. The author prefers stems with modular sleeves for the possibility of rotating the sleeve to optimally fill the metaphysis (Figure 1B).


Figure 1: Previous valgus osteotomy after hardware removal (A). After implantation of an S-ROM stem with the triangle of the sleeve in the greater trochanter (B).

Femurs that have had varus osteotomies are rare but more complex to manage (Figure 2A). After a varus osteotomy, the greater trochanter is moved more medially, which makes it difficult to implant a stem without damaging the greater trochanter and the insertion of the glutei muscles with a high probability of limping. Unless surgeons are willing to accept a varus position of the stem (which is known to be a mistake) both in cemented and uncemented situations, they should use a cemented stem pushed laterally during the insertion after passing the greater trochanter to avoid varus positioning. It is unfortunate that dysplastic femurs are small and that cemented stems are often unsuitable. They are unsuitable because leaving an adequate space for the cement mantle (2 mm circumferentially) causes th e stem to become too thin to resist the stresses, resulting in too much flexibility. The thinness and flexibility of the stem puts the cement-implant interface at risk, increasing the likelihood of aseptic loosening. Another contraindication in the use of a cemented stem is that the holes of the plate and the screws generally prevent an adequate pressurization of the cement.

The other option for femurs with varus osteotomies is to perform the osteotomy again and place the proximal femur in the original position. This automatically moves the greater trochanter laterally, enabling a surgeon to gain access to the femoral canal without damaging the greater trochanter and muscular insertion (Figure 2B). Performing this osteotomy usually requires cutting the femur slightly distal to the lesser trochanter and preparing the distal part in an antegrade fashion and the proximal part in a retrograde fashion after removal of the hardware (if these steps have not been performed in the previous surgery). Another option is preparing the metaphysis from above after dislocating the hip and cutting the femoral neck. Surgeons should remember to ream in a varus direction, cut the femur, and progress preparing the distal femoral canal.


Figure 2: Osteoarthritic hip with varus osteotomy (A). After valgus osteotomy with an S-ROM stem implanted (B).

The author has tried both options using a cementless stem to perform a valgus osteotomy in femurs with previous varus osteotomy. The author prefers antegrade reaming before performing the osteotomy because it is easier to handle the metaphysis when it is still attached to the distal femur. This also reduces the risk of fracturing the proximal fragment while reaming it retrograde. This risk is likely to occur because the bone is weakened by the previous osteotomy and may be fractured by the pressure of the bone clamps. To obtain perfect congruency at the site of the osteotomy, a laterally based wedge should be removed. This removal will shorten the femur that most of the times has already been shortened by the removal of a wedge during the first osteotomy. Thus, the author prefers not to remove the wedge and leave a medial gap to be filled by bone formation. After having had one delayed union, the author has used bone graft from the femoral head to enhance the healing of the osteotomy with no delayed unions since.

Conclusions

The management of version abnormalities and angular deformities in DDH requires accurate preoperative planning, the selection of suitable implants, and the use of appropriate surgical techniques. If these steps are taken, then the results of hip prosthesis are only reasonably inferior to the results obtained from primary osteoarthritic hips.

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Author

Dr Marega is from Casa di Cura S. Anna in Brescia, Italy.