Hip dysplasia is a complex developmental disease that primarily affects larger breeds of dog. The condition has a polygenetic mode of inheritance. Estimates of its hereditability vary from 0.2 to 0.6. Multiple factors influence and modify the disease, e.g., nutrition, growth rates, environment, exercise, muscle mass and hormone balance. The crucial event in the pathogenesis of hip dysplasia is the development of coxofemoral instability. The pathological changes which ensue are the result of natural attempts by the body to counteract the instability followed later by degenerative changes induced by abnormal load-bearing.

Pathogenesis

Affected animals have a normal hip at birth but the development of the supporting soft tissues fails to keep pace with skeletal growth and leads to loss of congruity between the femoral head and the acetabulum. Possible reasons for abnormal development of the hip include:

 Delayed ossification of the femoral head (Madsen, Reimann and Svalastoga 1991)

 Reduced hyaluronic acid concentration leading to joint laxity (Cullis-Hill 1985)

 Increased ratio of collagen type III in supportive tissue may be responsible for coxofemoral joint laxity ( Madsen et al.1994)

Development of the dysplastic hip and radiographic changes (Riser 1973)

The changes include:

 Primary joint instability.

 Increased synovial fluid production and thickening of the teres ligament.

 Early subluxation, thickening of the joint capsule and increased tension in the pelvic musculature.

 Impingement of the femoral head on the dorsal acetabular rim causes cartilage fibrillation, microfracture or cartilage erosion.

First indications of hip pain occur at about 5 months of age, radiographs at this stage confirm some degree of subluxation although the conformation of the femoral heads appears normal. Physical tests for laxity include the Bardens sign, Ortolani sign and Barlow sign.

The main factors predisposing to subluxation that are considered from radiographs particularly in assessing future treatment a re:

1.  Insufficient dorsal acetabular rim ("shallow acetabulum")

2.  Increased angles of inclination (coxa valga) and anteversion of the femoral neck, (normal angles: inclination 146 degrees, anteversion 27 degrees.)

From a clinical stand point dogs aged 5-9 months of age with hip dysplasia can be graded radiographically from 1-4 according to the of degree of subluxation, (secondary changes are often minimal at this stage):

 Grade 1 = mild dysplasia--50 % of the femoral head within the acetabulum

 Grade 2: 30%

 Grade 3: 10-20%

 Grade 4 = severe dysplasia--complete luxation of the femoral head

The use of stress radiographs is advocated by some authors ( Bardens, 1972, Smith et al, 1990, and Svalastoga, 1991) to demonstrate coxofemoral instability which might not be apparent on standard ventro-dorsal projections of the hips.

Maintenance of joint congruity in the early stages of instability is an important consideration in treatment as this can reduce the severity of subsequent changes in the dysplastic hip.

The response to instability--the joint capsule becomes thickened and periarticular osteophytes develop in an attempt to stabilize the joint, a spontaneous improvement in limb function usually occurs as the dog reaches maturity and the hips stabilize. Some 76% of dysplastic dogs will have no further lameness problems after 14 months of age.

In the long term the acetabulum becomes shallower and abnormal loading of the hip results in considerable remodeling of the joint and periarticular area with extensive periarticular osteophyte formation and secondary osteoarthritic change. Radiographically we notice that the degree of subluxation is reduced in the mature dog with dysplasia but the degree of periarticular new bone is increased. There is usually no significant change in periarticular osteophyte formation after 2 years of age. Dysplastic dogs with osteoarthritic change present may remain asymptomatic for several years and then start to show clinical signs of hip pain due to such factors as obesity, hip trauma, movement of joint mice and occasionally secondary infection.

Treatment options in the immature dog

It is well known that a spontaneous improvement in limb function often occurs as the dysplastic dog reaches maturity and the hips stabilize and only some 30% of these cases will require treatment in later life.

Treatment plan for the pup presented at 5-6 months of age with hip pain and lameness: Conservative management--encourage the hips to stabilize by limiting exercise to several regular 10 minute walks a day on a leash only for 6 weeks, use NSAIDS (Carprofen, Rimadyl Pfizer) if necessary to control pain. If the dog responds to conservative management then continue to control exercise until the pup is at least 12 months of age however after the first 6 weeks longer periods of exercise can be given (20-30 minutes at a time), swimming is also useful--the aim being to build up muscle mass to support the hips. Conservative management will give a successful outcome in 76% of cases (Barr et al 1986).

If the dog does not appear to be responding to this regime of conservative management within 6 weeks then surgical options should be considered, these include:

 Triple pelvic osteotomy

 Femoral or intertrochanteric osteotomy

 Excision arthroplasty

 Pectineal myectomy

Triple Pelvic Osteotomy (TPO)

This involves isolation of the acetabular segment of the pelvis by creating osteotomies in the pubis, ischium and ilium, prior to plate fixation, rotating the acetabular segment laterally to cover the femoral head more effectively ( Slocum & Devine,1986, Slocum& Slocum 1992). The indications for the procedure include:

 Patients between 5 and 9 months of age ( not exclusively)

 Clinical lameness (?)

 Little or preferably no, secondary radiographic osteoarthritis.

 Good integrity of the dorsal acetabular rim as assessed from the Ortolani test +/- a dorsal acetabular rim radiograph.

Published results for triple pelvic osteotomy tend to suggest that 80-90% of cases will regain satisfactory function within 3 months of surgery.

Intertrochanteric Osteotomy

Femoral osteotomy (Prieur 1987) involves removal of a medially based intertrochanteric wedge which allows the angle of inclination of the femoral neck to be reduced, thus pushing the femoral head further into he acetabulum. This technique is most appropriate when coxa valga is present with an angle of inclination in excess of 150 degrees. The procedure is used in dogs between 9 and 15 months of age. The osteotomy is stabilized with a special hook plate (Stratec) or a well contoured DCP. The technique particularly if the hook plate is used is demanding with respect to planning and execution and is performed less and less as TPO techniques have gained favour.

Femoral neck lengthening procedures

The aim of these is similar to that of intertrochanteric osteotomy but improved femoral contact is achieved by creating a vertical cut between the femoral head and the greater trochanter and then displacing the femoral head medially by driving wedges down into the osteotomy.

Pectineal myectomy

This technique appears to reduce pain in a dysplastic or osteoarthritic hip but does not have any further influence on the further development of radiographic signs (Bowen et al, 1972). Its indication in immature animals is as an adjunct to conservative treatment when osteotomy is not indicated but pain is difficult to control, significant improvement is reported in 78% of cases following surgery (Vaughan et al, 1975)

Excision Arthroplasty

This must always be considered a salvage procedure. In the skeletally immature animal it should be reserved for those cases where the signs are severe and non responsive to conservative management and when corrective osteotomies are inappropriate due to the degree of deformity, secondary changes or cost.

Delay surgical option until maturity

If the degree of hip dysplasia is too severe to consider TPO then ideally manage conservatively until dog is 11 months of age, when the dog is old enough for total hip replacement.

Mature dogs with hip pain

Conservative options: NSAIDS, nutraceuticals, exercise control, hydrotherapy. If non responsive then surgical options are total hip replacement or excision arthroplasty

References

1.  Bardens J.W. (1972), Palpation for the detection of dysplasia and wedge technique for pelvic radiography. Proceedings of the Annual Meeting of the American Animal Hospital Association, Las Vegas; 468-471.

2.  Barr,A.R.S., Denny,H.R., & Gibbs, C.(1987), Clinical hip dysplasia in growing dogs: the long-term results of conservative management. Journal of Small Animal Practice.: 28:243-52.

3.  Bowen J.M. Lewis, R.E. Kneller, S.K. Wilson, R.C. &Arnold, R.A.(1972), Pectineus myectomy in the dog. Journal of the American Veterinary Medical Association;161:899.

4.  Cullis-Hill. D. (1986),The role of hyaluronic acid in joint development and integrity. Veterinary Radiology; 27: 23, refs Abstract-1985 Scientific Meeting, Australian College of Veterinary Scientists.

5.  Madsen, J.S., & Svalastoga.E. (1991),Effects of anaesthesia and stress on the radiographic evaluation of the coxofemoral joint. Journal of Small Animal Practice ; 32: 64-68

6.  Madsen, J.S., Oxlund H.,Svalstoga E.,& Schwarz, P., (1994) Collagen type III:I in hip joints of dogs susceptible to hip dysplasia, Journal of Small Animal Practice, 35: 625-354

7.  Madsen, J.S., Reimann, I., Svalastoga.E., (1991) Delayed ossification of the femoral head in dogs with hip dysplasia., Journal of Small Animal Practice, 32: 351-354.

8.  Prieur, W.D.,(1987)Intertrochanteric osteotomy in the dog: theoretical consideration and operative technique. Journal of Small Animal Practice.;28:3-20.

9.  Riser W.H. (1973) The dysplastic hip joint: its radiographic and histologic development. Journal of the American Veterinary Radiology Society, 14: 35-50

10. Slocum, B. and Slocum, T.D. (1992), Pelvic osteotomy for axial rotation of the acetabular segment in dogs with hip dysplasia. Veterinary Clinics of North America, Small Animal Practice. 22: (3), 645.

11. Slocum B. & Devine, T.M (1986);.Pelvic osteotomy technique for axial rotation of the acetabula segment in dogs. Journal of the American Animal Hospital Association, 187: 828-33.

12. Smith, G.K., Biery, D.N., & Gregor, T.P. (1990) New concepts of coxofemoral joint stability and the development of a clinical stress-radiographic method for quantitative hip joint laxity in the dog. Journal of the American Veterinary Medical Association, 196: 59-70

13. Vaughan, L.C., Clayton Jones, D.C & Lane, J.G.,(1975), Pectineus muscle resection as a treatment for hip dysplasia in dogs. Veterinary Record 96:145-8