The Rationale for Surgery in Hip Dysplasia
World Small Animal Veterinary Association World Congress Proceedings, 2007
Simon C. Roe, BVSc, PhD, DACVS
North Carolina State University
Raleigh, NC, USA

While the actual cause of hip dysplasia is still unknown, the consequences of the subsequent laxity of the joint are fairly well understood. Radiographically, the damage progresses and the body responds, resulting in degeneration and remodeling of the joint. In most dogs, this process is accompanied by significant pain and progressive loss of range of motion. Surgical intervention has two potential roles--prevention of degeneration and reduction of the pain in the diseased joint.

Juvenile pubic symphysiodesis (JPS) can be used to alter pelvic morphology, and specifically the acetabular angle, in order to reduce subluxation events in a puppy with hip laxity that is likely to result in joint degeneration. This is done by surgically damaging the pubic portion of the symphysis so that, as the pelvis grows and expands, the pubis, on the ventral aspect of the acetabulum, does not, and the angle of the face of the acetabulum increases. This improves the "coverage" of the femoral head, and improves the functional laxity of the joints. In order for this procedure to really alter pelvis shape, puppies with laxity must be identified before they are 16 weeks of age, and before they have clinical signs. This can only happen if clients are advised of this option in hip dysplasia prone breeds when they are presenting for vaccinations. Because identification of laxity on palpation is challenging in an awake, wiggling, puppy, sedation and, usually, a distraction view radiograph are necessary to determine if a puppy has evidence of passive laxity that is severe enough to warrant the procedure. We perform a PennHIP study, and, because time is often short, guestimate the Distraction Indices (DI's). Clients are advised that DI's at this age are not truly predictive of later disease. In some puppies the DI's might reduce, while in others, they might increase. The Ortolani manipulation is also performed to assess functional laxity. Relocation angles greater than 35 deg, and subluxation angles greater than 25 deg carry a much more guarded prognosis, as, at 16 weeks, only 15 to 20 deg of rotation of the acetabulum is expected. The procedure is done at the same time as spay or neuter, and adds little time or risk. The cranial portion of the growth plate is exposed and the cartilage of the growth plate destroyed with electrocautery. Because we have had some cases in which the pubis did not appear to fuse, I have added a step to separate the pubis and ischial portions of the symphysis. Using narrow duck bill rongeurs, an osteotomy is performed between the most cranial aspects of the obturator foramen. The bone fragments are placed over the pubis to act as a bone graft.

Puppies identified with laxity after 16 weeks are potentially candidates for triple pelvic osteotomy (TPO). Again, identification of dogs with laxity prior to the development of clinical signs will improve the chances of this surgery producing a good result. TPO also aims to increase the "coverage" of the femoral head by rotating the acetabulum. After osteotomies of the pubis, ischium and ilium, the acetabular fragment is rotated, and held in that position with a plate with a built-in rotation angle between the proximal and distal portions. In most cases, a 20 deg rotation is used, as more than that does not seem to improve coverage, and increases the possibility of pelvic narrowing. Selecting the right candidate is the most contentious issue with this surgery. A conservative surgeon (like myself) will only give a good prognosis if the hip has minimal to no damage. This is determined by a combination of palpation and radiographs. The angle of reduction and subluxation are estimated--greater than 30-35 degrees is a concern. Also, if these angles are close to one another (< 5 deg), this suggest rim damage. The state of the dorsal acetabular rim is reflected in the "crispness" of the reduction--a crisp drop suggests minimal rim damage; a soft, sliding feel suggest damage and "rounding". The feel of the return into the acetabulum is also assessed. A soft landing suggests that there is already round ligament and ventral capsule proliferation beginning to fill the acetabulum. Radiographically, dorsal acetabular rim (DAR) health can be assessed by observation of the dorsal acetabular edge (DAE) on a good quality ventrodorsal view. It should be nearly straight, but, as wear progresses, it develops a more curved appearance. The DAR can also be assessed using a specific view that is aligned with the long axis of the pelvis, though the portion assessed is somewhat caudal to the region of maximal damage.

A less conservative surgeon will accept some damage in the hip, with the feeling that TPO will eliminate functional laxity, and, thus, continued damage. Although some degenerative change is still expected, clinically, these dogs may do well for the rest of their lives, because of the improved hip articulation. Long term studies are needed to further study these cases.

TPO carries a moderately high rate of complications, though most are not considered serious enough to warrant another surgery. Screw loosening, with subsequent movement of the acetabular fragment, sometimes with pelvic canal narrowing, is common. It can be reduced by ensuring that the proximal screws engage the sacral wing and body. One surgeon has suggested that the screws be assessed 2 weeks post-op, and, if loosening is evident, the screws are tightened under fluoroscopic guidance. In high risk patients (i.e., big and heavy), a ventral plate adds significantly to the strength of the repair.

Surgical management of hip pain from either acute or chronic disease entails either femoral head and neck ostectomy (FHO) or total hip replacement (THR). The timing of these surgeries must be significantly influenced by the client's perception of their dog's quality of life, balanced with the expected benefits and complications of the surgery. Most clients must feel like medical management has failed before considering a surgical option. Weight loss, activity modification, rehab therapy, nutritional and pharmaceutical approaches can be very successful in many dogs. Having said that, many clients aren't good with weight management, don't want to modify their dog's activity, and feel that continuous drugs are not worth it--they arrive at the surgical option sooner than others.

THR is a moderately complex procedure that requires a good quality facility and a dedicated team to achieve consistently good outcomes. There are a number of different prostheses available, and the choice depends mostly on surgeon preference. The primary categories are cemented and uncemented. There are a number of cemented systems available. Within uncemented, there is a press-fit, ingrowth system and a screw-fixed system. My preference in most dogs is for the ingrowth system (BFX®, BioMedtrix, NJ). This developed from years of clinical experience with a research system, followed by a number of years using a cemented system. Long term implant stability is superior with the ingrowth system, so I feel very comfortable replacing the hip in 1 yr old dogs. Recently, we've done two 7 month old dogs, as they had spontaneously luxated, and would not have been good candidates in 3-4 months due to femoral malpositioning.

THR involves removal of the femoral head, preparation of the femoral canal for the stem of the prosthesis, and preparation of the acetabulum for the cup. The femoral stems are either titanium or chrome cobalt steel. In the cemented systems, the cup is only plastic (ultra high molecular weight polyethylene), but in the uncemented systems, there is a metal shell into which the plastic bearing surface is fitted. Most systems have some modularity to the metal femoral head component, allowing the surgeon to optimize the "tightness" of the hip. In cemented hip surgery, the bones are prepared, the cavities filled with liquid polymethylmethacrylate (PMMA), and the implants positioned and held until the PMMA has set. Long term success requires that the interfaces between the PMMA and the implants, and the PMMA and the bone, stay solid and stable for the life of the patient. Many factors influence the longevity of this interface.

In the BFX® system, the bone beds are prepared with much more precision. The implants are hammered into the bone until they are firm. Over the following 3 to 4 weeks, bone grows into the porous surface, creating a biological interlock.

The primary complications associated with THR are luxation, infection, fracture and loosening. Luxation is primarily craniodorsal. Young active patients with very lax hips seem more at risk. The orientation of the cup and the relative versions of the cup and stem seem to influence stability also. The primary risk period is in the first 4 weeks after surgery. Once the capsule has healed and the muscles returned, luxation is an uncommon event. Infection is a risk with any surgery, but, in THR, it usually requires removal of the implants. Our patients must have no active infections (skin, ears, mouth) at the time of surgery. We take many extra measures to ensure sterility--intensive room cleaning, limited personnel movement, barrier draping, separate approach instruments, multiple glove changes and antibiotic flush. Infection seems less likely in uncemented hip replacement.

Fracture can occur intra-operatively, or post-operatively. Intra-operative fissures with the uncemented system are usually a result of slight misalignment of the broach during femoral preparation. Some fissures have occurred during stem placement, particularly when the implant fills the canal, and is in contact with cortical bone. In most cases, placing double loop cerclage around the proximal femur has provided sufficient stability for the bone and stem. Post-operative fissures can result in femoral stem subsidence and rotation or complete fracture. These are complex to manage, requiring creative plate application, and sometimes conversion to a cemented stem due to bone damage.

Long term loosening was not seen in the ingrowth system we used in the 1990's. Our current system has only been in patients for 4 years, but we are hopeful for similar performance. Critical analysis of cemented implants suggests interface changes are common at 5 years, though many dogs do not have clinical signs.

Cemented hip replacement is still a very good option, and I use it in older dogs, particularly if they have thin cortices. A Micro-hip system has recently been introduced (BioMedtrix) and allows THR to be considered in dogs as small as Miniature Poodles.

THR greatly improves patient mobility and comfort, even when only one hip is replaced. We replace both hips in about 20% of cases. These are usually working dogs, with a high athletic expectation, or giant breeds.

FHO is still a good option for reducing hip pain when medical therapy has failed, and THR is not available or affordable. The function of the limb will not be as good in bigger, heavier dogs, but they will nearly always be more comfortable. It is important to set the expectations of the client appropriately, as many are hoping that the surgery will make their dog normal. Success in FHO requires total removal of the femoral head and neck. I work hard with rongeurs and rasp to remove anything that protrudes after I have made the cut. In some cases, removal of large osteophytes on the dorsal acetabular rim can help with the development of a comfortable pseudo-articulation. My preference is to interpose a portion of the deep gluteal muscle between the femur and acetabulum during close, by suturing the tendon of the deep gluteal to the lesser trochanter area. Post-operatively, rehabilitation therapy can be very helpful in building the supporting musculature, and creating a "joint" with good range of motion.

Speaker Information
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Simon C. Roe, BVSc, PhD, DACVS
North Carolina State University
North Carolina, USA

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