Hip Dysplasia & THR Surgery

What are Canine Hip Dysplasia and Osteoarthritis?

Hip dysplasia, meaning abnormal growth of the hip, is a very common problem in dogs. One in three dogs, i.e. more than 25 million in the US, suffer from Canine Hip Dysplasia (CHD) and over 5 million of those have progressed to severe, degenerative, hip joint disease. Half a million new cases enter this pool every year.

A normal hip is a tightly joined “ball and socket” joint. The normal hip joint enables the hind leg to move smoothly in a full range of motion and allows a dog to rise, walk, run, jump, and turn effectively. In the diseased hip, the “ball and socket” become irregular, rough, and deformed, causing pain and decreased range of motion.

The irregular nature of the joint will contribute to the development of Osteoarthritis. Osteoarthritis (OA) also known as degenerative arthritis, degenerative joint disease or osteoarthrosis, is a group of mechanical abnormalities involving degradation of joints, including cartilage and bone. Symptoms of OA include joint pain, tenderness, stiffness, locking, and swelling.

Dysplastic hips are painful and lack smooth movements; therefore the dog’s quality of life can be severely affected. Selection of the appropriate treatment option for this condition is dependent on many factors such as: age, severity of hip dysplasia, development of osteoarthritis, degree of pain/discomfort, and owner’s expectation and financial ability.[1]

What are the Signs?

Dogs with chronic joint disorders still wag their tails, and owners frequently fail to register the subtle signs of constant pain.

Dogs with hip dysplasia are often described as, “lazy,” but inactivity, especially in young dogs, is often a consequence of chronic pain and discomfort.

Because dogs walk on four limbs, they can compensate for this pain by shifting weight to their front limbs. This adaptation can result in compensation injuries in the elbow and shoulder.

What are the Treatment Options?

Fortunately, not every pet diagnosed with CHD requires treatment. Many dogs with pain and lameness associated with hip dysplasia can be effectively managed with conservative methods. Conservative methods include: weight management, moderation of excessive exercise/activity, providing warm comfortable bedding, and the use of non-steroidal anti-inflammatory drugs, physical rehabilitation, and oral supplements as needed.

If your companion is in severe hip pain and the quality of life is decreased, and conservative methods are not effective, surgical treatment should be considered.

Treatment options are dependent on age, body weight/size, breed, degree of discomfort/dysfunction, physical examination, and radiographic (X-ray) findings, your expectations for your companion and your budget.

For young pets diagnosed with CHD, Juvenile Pubic Symphysiodesis (JPS) or a pelvic osteotomy, Double Pelvic Osteotomy (DPO) or Triple Pelvic Osteotomy (TPO), can be performed. These techniques aim to modify the growth and/or shape of the pelvis to better accommodate the head of the femur and provide more normal joint function. Unfortunately, these procedures can only be performed in a fairly narrow age window, so most patients miss the opportunity to be treated by these methods.

For mature patients, Femoral Head Ostectomy (FHO) and Total Hip Replacement (THR) surgeries can be indicated.

FHO involves the removal of the ball portion of the “ball and socket” hip joint. The head of the femur is removed and, in time, scar and soft tissues fill the joint space and the muscles around the hip develop and compensate for the lack of a hip joint. This will eliminate a great deal of the pain associated with CHD, but it will not maintain the normal function of the hip. For this reason, FHO is, generally, not recommended for large breed patients.

THR surgery replaces a painful and dysfunctional hip joint with an artificial prosthesis, in order to provide a pain-free, fully functional joint. Total hip replacement can also be considered for a dog with a painful and abnormal hip due to fracture, luxation (dislocation), or necrosis (severe degeneration) of the femoral head. The THR prosthesis replaces both the ball and socket. There are a variety of THR implants on the market. In the next section we will cover Canine THR in greater detail.

What is the Zürich Cementless THR Innovation?

Over two decades, KYON has developed, and fine-tuned a unique Total Hip Replacement (THR) system, now applied in over 10’000 cases. Worldwide, about one in three of the 500 veterinary surgeons instructed by KYON to perform THR have gone on to practice it, contributing to about 1’200 procedures performed annually.

When trying to design an implant for veterinary use, one is confronted with some serious logistical challenges.
Chief among these are:

    – Patient Compliance
    – Biomechanical Function
    – Age & Expectations for Performance

Patient Compliance

When it comes to Doctor Patient communication regarding post-operative compliance, obviously we have some issues… You can’t tell a canine patient, “Ok, Mr. Fido, for the next 6-8 weeks, I want you to keep your leg elevated, be sure to use crutches when getting around the house, obviously, no jumping on or off the sofa, counter surfing, running, chasing the cat, or rough play with your brothers and sisters…” Because of this, we have to design implants to be immediately and indefinitely stable, minimally invasive, and rapid healing.

THR systems fall into two main categories: Cemented and Cementless.

We like cementless because cement can be difficult to handle, requires some pretty nasty chemical compounds for mixing, and has been shown to cook and kill surrounding bone. Additionally, once the cemented stem is in place, it is a very rigid construct, surrounded by a layer of dead bone and then living bone which goes through, what is called, “cyclic loading.” This is the fluctuating and repetitive stress that living bone undergoes during normal use. The fixed cement component, when subjected to this fluctuating stress, is prone to loosening, bone resorption, femur fracture, and stem breakage.

Traditional cementless stems are “press-fit.” This is pretty much how it sounds. You remove the head of the femur and then clear the center of the femur for the stem and hammer or “press” the femur into the femoral canal. In time, bone will grow onto the surface of the stem and become fixed. This is the time when you need “Mr. Fido” to be compliant, a real challenge. Also, when the stem is pressed-in, it becomes coated in fatty marrow which makes it difficult for bone to grow onto the prosthesis, lengthening the time it takes for this adhesion to occur. If the implant does not incorporate into the bone, the implant will gradually loosen and become painful. Implant loosening is one of the primary complications in human and animal THR patients.

Unlike traditional cementless THR systems, the KYON Zürich Cementless THR uses screws that are locked into both the stem and the interior wall of the femur to anchor the stem so that, on day 1 of surgery, the implant is stable. Because the stem is anchored to only one wall of the femur, the stem moves with the bone, avoiding the fatigue of cyclic loading. While this does ease some of the patient compliance requirements, you still need to follow your surgeons post-op care instructions dutifully.

To speed the healing process and prevent against infection, the implants are manufactured from titanium and designed to provide surfaces that encourage osteo-integration. To facilitate the process of integration the stem is plasma coated with pure titanium and the cup consists of inner and outer titanium shells that prevent bone from contacting the polyethylene liner, while facilitating boney on-growth.

Biomechanical Function

If you have ever seen a dog scratch it’s ear or leap over a 5 ft. fence in one bound, I think you will understand that, when it comes to performing THR in dogs, we have our work cut out for us. The design objective for articulation (movement) is to maximize the range of impingement-free motion by minimizing the size of the head-neck component, with maximum coverage of the head by the cup component. This objective was met in steps, with the final optimization of the neck shape to reduce stress concentration and the Ultra High Molecular Weight (UHMWPE) cup inlay providing approximately 200° of cover. The combination of these two components allows for over 120°, now 135° with our larger head/cup sizes, of impingement-free angulation. The average luxation (dislocation) rate is about 5% with several surgeons reporting it as low as 1%. Very young – 6-10 months – dysplastic dogs present an even greater challenge in meeting stability criteria, because they show a great range of motion, and therefore an increased risk of luxation. A series of intra-operative stability tests, and appropriate corrective measures, if any lack of stability is detected, provide a strong guarantee against postoperative luxation.

Age & Expectations for Performance

Unlike humans who tend to have THR performed later in life when their expectations for mobility are quite limited, canine THR patients are typically between 1.5 – 3 years of age. The implants need to perform for the life of the patient, including the most active years.

The stem achieves permanent anchorage on the femoral side through bony ingrowth from the medial cortex without coupling to the lateral cortex. Stability required for ingrowth is guaranteed by locking screw fixation of the stem to the cortex from the inner side of the bone**. This results in near physiological loading of the proximal femur, i.e. absence of stress shielding.

On the acetabulum side, the outer shell of the cup is manufactured from perforated, highly compliant, titanium, with an inner non-perforated shell[3] and an ultra-high molecular weight polyethylene (UHMWPE) inner lining to receive the head of the stem. The double-shelled design** provides for rapid and consistent integration of the acetabular bone into the outer shell of the cup.

Minimizing wear reduces the risk of bone lysis mediated aseptic loosening. Instead of a conventional spherical shape, an artificial Fossa™** provides an articulating surface that minimizes contact with the femoral head and improves hydrodynamic lubrication within the cup. This modified geometry of the contact area between the head of the prosthesis and the polyethylene liner reduces the contact stresses several fold. A new amorphous diamond-like coating (ADLC) provides increased hardness and lubricity to the head/cup interface, also reducing wear.

**KYON has an exclusive license for use in the veterinary field of the related patents from Scyon Orthopaedics, AG, Au, Switzerland.
In addition to the design, the choice of materials is also an important factor here. We use titanium for all components

Notes:
[1] – Hayashi, Kei, DVM, MS, PhD, DACVS – Total Hip Replacement In Dogs (ACVS)
http://www.acvs.org/AnimalOwners/HealthConditions/SmallAnimalTopics/TotalHipReplacement(THR)inDogs/

[2] – Annual Report 2002, The Swedish National Hip Arthroplasty Register, Dept. of Orthopaedics, Sahlgrenska University Hospital, April 2003, www.jru.orthop.gu.se

[3] – Skurla CP, Pluhar GE, Frankel DJ, Egger EL, James SP – Assessing the dog as model for human total hip replacement. Analysis of 38 canine cemented femoral components retrieved at post-mortem.
J Bone Joint Jurg Br. Jan;87(1):120-7, 2005
VetSurg 38: 1-22, 2009 (p.8)
[4] – Tepic S, Remiger AR, et al., – Strength Recovery in Fractured Sheep Tibia Treated with a Plate or an Internal Fixitor: An Experimental Study with a Two-Year Follow-up,
J Orthop Trauma 11(1):14-23, 1997
[5] – Vuillemin T, Raveh J, Sutter F, – Mandibular Reconstruction with the Titanium Hollow Screw Reconstruction Plate (THORP) System: Evaluation of 62 Cases
Plast Reconstr Surg 82(5):804-14,1988

This page is provided for general information purposes only. This information should only be used for basic educational purposes and should not be used for diagnostic purposes. Consult with a veterinary professional about treatment options that may be appropriate for your pet’s medical condition.

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