Kapitel 59 - Surgical Management of Hip Dysplasia

Question 1

List prophylactic, savage and palliative procedure of the hip joint

Question 2

At what age was the greatest change (improvement) noted in puppies undergoing juvenile pubic symphysiodesis?

Answer 2

At 12 weeks of age

Question 3

Which individuals are generally considered good candidate for pelvic osteotomy?

Answer 3

Dogs with laxity of the hip joint, consistent with early stages of hip dysplasia, but without radiographic evidence of secondary osteoarthritis or complete luxation.

Question 4

At what age is the pelvic osteotomy advised?

Answer 4

In general, younger than 1 year

Question 5

What are the angles of the rotation on the plates for pelvic osteotomy?

Answer 5

Ranging from 20-45
- 20, 30 or 40 degrees are most commonly used for triple osteotomy

• 25 or 30 degrees for double

Question 6

What is the desired degree of acetabular rotation in pelvic osteotomy?

Answer 6

5 degrees greater than the measures angle of subluxation.

Question 7

What are the most common complications in pelvic osteotomy?

Answer 7

Screw loosening and pelvic canal narrowing

Question 8

Which material used in THR have highest elastic modules and why is this important?

Answer 8

a. Stainless steel and cobalt-chromium (200 GPa) compared to titanium (100 GPa)

b. The greater the elastic modulus mismatch between apposing surfaces, the greater the risk for development of wear debris or stress shielding

Question 9

Shortly describe adhesion, abrasion, erosion, corrosion and fatigue wear

Answer 9

a. Adhesive wear occurs when material from the softer bearing surface is transferred to the opposing surface and breaks free from its original surface.

b. Abrasive wear occurs when an irregularity on a hard surface damages the opposing surface

c. Fatigue wear is the consequence of cyclic loading that results in the development of surface or subsurface microcracks.

d. Erosive wear can be the result of either solid particle erosion or impingement. Solid particle erosion is the loss of material resulting from the impact of solid particles on a surface. Particles that are suspended in the articulating fluid can accelerate, decelerate, or change direction, resulting in impact. Impingement wear, as it pertains to hip replacement, is generally a consequence of implant-implant contact or implant-bone contact during functional range of motion.

e. Corrosive wear is damage to the surface due to a chemical reaction. specifically galvanic corrosion, is the result of oxidation that generally results from the interaction of dissimilar metals.

Question 10

Which is the only commercially available articulation combination in veterinary hip prothesis? And what are the major problems associated?

Answer 10

• Metal-on-polythylene
• Wear debris, secondary osteolysis and aseptic loosening

Question 11

What are the broad categories of fixation systems used in THR?

Answer 11

a. Cemented
- Medical graded bone cement is Polymethyl methacrylate(PMMA = akrylplast)
- Easier preparation of the bones
- generally strongest within 2 days of implantation, but then may weaken and loosen over time because the cement cannot adapt to changes in the bone or alteration of loads.
- Cemented have a higher risk of aseptic loosening.

Cementless
- Are either press fit–ingrowth, screw in–ingrowth, or stabilized by monocortical screws
- generally rely on bone ingrowth (osseointegration) or bone ongrowth for long-term stability. Ingrowth occurs with pore sizes between 50 and 400 µm with a micromotion at implant-bone interference of less than 20 µm
- A porosity of 30% to 40% (voids:material) is ideal to permit ingrowth but avoid weakening of the implant surface.
- Plasma spraying (used in Zurich Cementless implants) or sandblasting implant (used in Helica implants) surfaces is used to promote ongrowth.
- All canine cementless acetabular cups are press fit–ingrowth, meaning that initial stabilization is achieved by impacting the cup into an acetabular bed that has been prepared to have a smaller diameter than the cup, resulting in very high friction between the cup and bone after it has been forced (impacted) into the bone bed

c. Hybrid (combination of the two)

Question 12

What systems are commercially available in Veterinary THR?

Answer 12

a. Biometrix
- BFX (pressfit, cementless)
- CFX (cement)
- Made of cobalt-chromium alloys. Titanium alloy implants are also available for BFX system.

b. Zurich cementless (Kyon)
- Uses a combination of press-fit (acetabular component) and locking screw (femoral component) fixation
- Made from titanium alloys.

c. Helica system (Veterinary instrumentations)
- is a “stemless” implant, and both femoral and acetabular components achieve immediate fixation via a screw-in design with self-tapping helical threads.

Question 13

What is the femoral inclination angle? What is the normal value and what is the term for a lower and higher neck angle?

Answer 13

• The angle between the femoral neck axis and the anatomical axis of femur in the frontal plane.
• 120-135 degrees (130-145 in chapter 60)
• A lower femoral neck angle is said to be a varus neck (coxa vara), whereas a higher neck angle is a valgus neck (coxa valga)

Question 14

Define the Femoral Offset

Answer 14

• The distance between the center of rotation of the femoral head and the anatomical axis of the femur. The femoral offset lateralizes the proximal end of the femur away from the pelvis. Conversely, the shorter the neck, the lesser the lever arm and the greater the force necessary to induce luxation, but there is also a greater risk of impingement.

Question 15

What are the contraindications for THR?

Answer 15

Concurrent orthopedic or neurologic disease (An exception may be a slowly progressive form of degenerative myelopathy).

Question 16

List the mechanical and biological complications associated with THR

Answer 16

Mechanical:
- Luxation
- Fractures of femur or acetabulum
- Acetabular cup avulsion
- Femoral stem avulsion
- Subsidence of the femoral stem
- Implant failure and cement failure

Biological
- Aseptic or septic loosening
- Stress protection

Question 17

What are the indications for THR?

Answer 17

• Management of the pain and loss of function associated with degenerative joint disease that is secondary to hip dysplasia
• The procedure has also been performed for avascular necrosis of the femoral head, capital femoral physeal fractures, femoral head fractures, acetabular fractures and poor outcomes following femoral head and neck excision, triple/double pelvic osteotomy, or darthroplasty

Question 18

What are some factors what can cause luxation after THR?

Answer 18

• Excessive anteversion (craniodorsal lux) or retroversion (caudoventral lux)
• Excessive or low angle of lateralization
• Preexisting laxity of the hip joint
• Periarticular osteophytes that impinge on the femoral neck
• Amputation of the contralateral limb results in the patient placing the total hip arthroplasty limb in greater adduction, which effectively creates a more open cup.
• conformational issue is the variation in standing angles for different breeds

Question 19

Describe the Vancouver classifications system used for fractures associated with total hip arthroplasty

Answer 19

• AG and AL correspond to fractures at the level of the greater trochanter or lesser trochanter
• B are those that involve the prosthesis
  a. B1 –> stable prosthesis
  b. B2 and B3 –> unstable prosthesis
• C describes fractures distal to the prosthesis

Question 20

What is the primary cause of aseptic loosening?

Answer 20

Wear debris

(Aseptic loosening has also been described as “wear debris–mediated osteolysis,” which more accurately describes the cause of the loosening)

Question 21

What is the incidence of Sciatic neurapraxia?

Answer 21

• The incidence in one study was 1.6%.

The neurapraxia is self-limiting and generally resolves within 6 weeks.