Hip luxation is the most frequently reported traumatic luxation in small animal practice. The joint capsule, the ligament of the femoral head and variable amounts of the gluteal muscle masses are ruptured. Damage to the articular surface of the femoral head or acetabulum may be present. Approximately 50% of cases will have associated major injuries, often chest trauma. The goals of treatment are to reduce the luxation with minimal additional damage to the articular surfaces and to stabilise the joint sufficiently to encourage soft tissue support. The high frequency of recurrence after closed reduction often results in surgical intervention. A number of surgical options are currently available.
Anatomy of the Hip Joint
The hip joint is classically referred to as 'a ball and socket joint' and involves the femoral head and acetabulum of the pelvis. The normal acetabulum has a semi-lunar articular surface which is open caudoventrally at the acetabular notch. The ligament of the femoral head originates from a non-articulating fossa located within the centre of the semi-lunar notch. This large ligament inserts on a non-articulating fovea at the centre of the femoral head. The primary stabilising features of the hip joint are the ligament of the femoral head, the hip joint capsule and the dorsal acetabular rim. The surrounding muscle tissue provides secondary support.
Luxation of the hip is described according to the displacement of the femoral head relative to the acetabulum. In most cases, the strongest pelvic muscles, gluteus medius and iliopsoas, pull the greater trochanter in a craniodorsal direction, leading to a craniodorsal luxation. Dorsal, caudal, ventral and rarely medial luxations of the femoral head have been described. Concurrent fractures of the femoral head and/or acetabulum have been described with traumatic hip luxations. These concurrent fractures will significantly influence treatment options. Pre-existing hip dysplasia is another important condition that will influence management of hip luxation.
Diagnosis of Hip Luxation
The history will usually indicate an acute onset of hindlimb lameness generally associated with recent trauma. Limb carriage is typically abnormal. The affected limb is externally rotated with the stifle joint pointing in an outward direction in craniodorsal luxations. Pain and crepitus are present on gentle manipulation of the hip joint. The lack of pelvic bone symmetry is a helpful indicator of hip luxation. Bilateral comparison of hindlimb length, with the limbs held in extension, may indicate hip luxation. Radiographs of the pelvis will confirm hip luxation. The presence of concurrent orthopaedic diseases must not be missed. The presence of hip dysplasia will significantly influence the management of hip luxation. Fractures of the femoral head, physis and/or acetabulum must be detected if present.
Closed reduction should be attempted as soon as the animal is physiologically stable. A period of 24 to 72 hours is ideal. This reduces muscle contracture, organisation of the debris within the acetabulum, and limits damage to articular cartilage due to the lack of synovial fluid nutrition. A general anaesthetic is always required. The manipulation required is very dependent on the location of the luxated femoral head: craniodorsal; caudodorsal; medial or caudoventral. The absence of a palpable 'click' may indicate the presence of soft tissue material within the acetabulum/and or concurrent injuries. This soft tissue may be part of the surrounding joint capsule that has become lodged between the femoral head and acetabulum or a haematoma located within the acetabulum as a result of tearing of the ligament of the femoral head. This soft tissue may prevent the femoral head from returning to its original deep position. Manipulation of the femoral head through a range of motion may facilitate displacement remnants of joint capsule, blood clots and fibrin. Reduction of the femoral head must be confirmed radiographically. Bandages can be applied to the hindlimb after closed reduction to reduce the risk of re-luxation of the femoral head. The Ehmer sling is frequently used, but it is difficult to apply correctly and complications have been reported. Radiographs should confirm that the femoral head remains reduced after bandage application. Hindlimb hobbles are another option. Re-luxation of the femoral head has been reported in 50-65% of referral cases. Closed reduction of caudoventral cases tends to be more successful.
Surgical intervention is indicated when closed hip reduction is not possible; when closed hip reduction results in repeated luxations, or when concurrent fractures (femoral head, acetabulum or other limbs) are present. A number of surgical techniques have been described. The craniolateral approach to the hip joint provides adequate exposure for most techniques. In chronic cases, or where additional exposure is required, an osteotomy of the greater trochanter may be indicated. Surgery allows assessment of soft tissue and orthopaedic structures needed to stabilise the hip luxation, namely, the hip joint capsule and articular surface of the femoral head. The quantity of remaining joint capsule is carefully assessed and preserved. Small femoral head/acetabular bone fragments are removed and large fracture fragments are reduced and stabilised. Significant damage to the articular surface of the femoral head may dictate a total hip replacement or a femoral head and neck excision arthroplasty. Debridement of the hip joint is then carefully carried out.
In some hip luxation cases, suturing of the joint capsule may be enough to prevent re-luxation of the femoral head. This occurs infrequently, and always involves acute cases with mild soft tissue damage. Instead, additional surgical techniques are often required to maintain reduction of the femoral head.
1. Toggle pin: This technique is based on replacing the ligament of the femoral head. A small stainless steel metal rod with an attached length of non-absorbable suture material is anchored in the acetabulum. The length of suture material is passed through the femoral head and tied laterally at the level of the greater trochanter. This technique provides temporary support until adequate soft tissue repair supports the hip joint. Suturing or reconstruction of the joint capsule is also performed in unstable cases. Both techniques should be performed where possible. Hobbles are placed between the hindlimbs to reduce hip abduction.
2. Dorsal prosthetic capsulorrhaphy: The joint capsule plays a crucial role in hip stability. A combination of a bone tunnel, bone screws and suture materials can be used to reconstruct the joint capsule if inadequate amounts are present. This technique is often used to support the toggle pin technique. It carries the disadvantage of requiring an osteotomy of the greater trochanter to access the dorsal acetabular rim.
3. Extracapsular iliofemoral suture: This technique supports a reduced femoral head using suture material placed through a bone tunnel at the origin of the rectus femoris muscle on the ilium, which is then passed through a second bone tunnel in the greater trochanter. No additional surgical exposure is required to perform this technique which is often used to support toggle pin repair.
4. Transarticular pin: This technique attempts to replace the ligament of the femoral head using a small diameter stainless steel pin/k-wire. Transarticular pinning is restricted to small-sized patients and requires a second surgery to remove the implant in 2-3 weeks after the initial procedure. Pin breakage or migration are potential complications.
5. Total hip replacement: Total hip replacement may be considered in cases with femoral and/or acetabular fractures, chronic hip dysplasia or chronic hip luxation.
As with any orthopaedic procedure, post-operative management of hip luxation is absolutely crucial. Indeed, successful reduction of the femoral head is often dependent on intact suture materials during the initial recovery period. Consequently, post-operative bandaging is strongly encouraged--hobbles are very helpful. These bandages are generally removed after 2-3 weeks. Strict cage confinement for 4-6 weeks is also advised with ten minute lead walks three to four times daily for toileting purposes. The prognosis following surgical management of hip luxation is generally good. Outcome is very dependent on the level of damage to the bones and soft tissues involved. Cases that are reduced early with adequate stabilisation carry a good prognosis and a good clinical function can be expected in 70-80% of cases. Success of surgical techniques are reported to be 85-100%. Degenerative joint disease is inevitable and long-term medical and lifestyle management is encouraged.