Clinical History
The cat was presented not able to weight bear on it's left hind leg, the owner having reported the cat coming in lame that morning. A clinical examination revealed a wound on the stifle, but palpation of the limb was not possible due to the cat's discomfort. A clinical examination of the cardiovascular and respiratory systems revealed no abnormalities and the cat was admitted for radiography under general anaesthesia. The cat had not eaten that morning, but the owners were warned of the risks of general anaesthesia and signed a consent form for general anaesthesia and possible surgery. The cat was premedicated with 0.16 mg acepromazine (ACP C VET) and 7.5 mg pethidine (Pethidine, Arnolds), both by intra- muscular route, 40 minutes prior to induction.

Radiography
Induction was achieved using 50 mg thiopentone sodium (Intraval, Rhone Merieux) by intra-venous route and an uncuffed 5 mm endo-tracheal tube used to intubate. Maintenance of anaesthesia was achieved using an Ayres T piece with flow rates of 2 L nitrous oxide and 1 L oxygen with halothane as the volatile agent. Antibiotic cover was provided with 100 mg ampicillin LA (Amfipen LA, Intervet) and additional analgesia with 15 mg carprofen (Rimadyl, Pfizer), both by subcutaneous route.

Radiographs of the pelvis were taken in both right lateral and ventrodorsal projections, and on examination the ventrodorsal projection showed that the left femoral neck had fractured with the femoral head remaining in the acetabulum. See Radiograph 1. The surgeon decided to perform an excision arthroplasty and the owners were contacted to advise them of the necessary surgical procedure.

Surgical preparation
A 22 g intra-venous catheter was placed aseptically into the right cephalic vein and an infusion of Hartmanns was started at a rate of 10mls/kg/hr. The left hind leg was clipped and prepared as in the first paragraph of Appendix 1, to include an area extending from the hock to the whole hip area, the incision site being over the greater trochanter of the femur. The cat was then moved to the theatre and final preparation of the skin took place as in the second paragraph of Appendix 1.

Surgery
The limb was lifted up by the nurse (holding the foot). The surgeon placed the first drape under the limb, lying along the ventral abdomen and left groin. A second drape was held under the limb and the nurse released the foot onto the drape. This drape was then wrapped around the distal limb and held in place with a towel clip. A third and fourth drape were placed to leave the proximal limb, including the hip exposed. See above.

The instruments used for this procedure included a standard kit, as described in Appendix 2. Additional instruments included Gelpi retractors, a Hohman retractor, an osteotome, an orthopaedic hammer and a disarticulator. Sterile saline was also used for flushing the surgical site.

An incision was made through the skin 5 cms in length over the greater trochanter, proximally just short of the dorsal midline and distally following the cranial border of the femur. The skin was undermined and retracted with the subcutaneous tissues. An incision was made in the superficial leaf of the of the fascia lata and the insertion of the tensor fascia lata muscle, following the cranial border of the biceps femoris muscle.

The biceps femoris was retracted caudally, and the fascia lata and tensor fascia lata retracted cranially, after separation of the tensor fascia lata from the middle gluteal muscle. Blunt dissection along the neck of the femur with the finger tip allowed visualisation of the deep gluteal muscle. The middle gluteal muscle was retracted caudally and partial tenotomy of the caudal two thirds of the deep gluteal muscle was performed. The joint capsule was incised and a Hohman retractor used to elevate the neck of the femur, whilst rotating the stifle outwards. An osteotome, with the orthopaedic hammer, was used to excise the neck of the femur. The excision site was checked to ensure that there were no sharp edges. The disarticulator was then used to sever the round ligament and remove the femoral head from the acetabulum. The area was flushed with sterile saline and the gluteal muscles coapted with 3 m chromic catgut (Ethicon) using mattress sutures. The fascia lata muscles were repaired as above and the subcutaneous tissues closed using 3 m catgut in a simple continuous pattern. The skin was closed using 2 m polyamide (Nylonamide, Animus) in a simple interrupted suture pattern. A total of 200 mls of Hartmanns was infused for maintenance, as no major haemorrhage had occurred. The endotracheal tube was removed within 3 minutes of the cessation of anaesthesia.

Post-operative progress.
The cat was in sternal recumbency within 15 minutes after extubation. Analgesia was repeated after 6 hours with 0.8 mg of morphine (Morphine Sulphate, Evans) by intra-muscular route.

The animal was sent home in the afternoon with instructions to the owner that the cat should be confined to house rest, and that revisits were necessary in 4 and 10 days for a post- operative check and sutures out. The cat was prescribed Rimadyl 20 mg tablets to be given half daily with food for 4 days, starting the following day, and Amfipen 50 mg tablets to be given one twice daily for 2 days, starting in two days.

The post-operative check was satisfactory - the cat was weight bearing on the limb, and sutures were removed after 10 days. Within 4 weeks the cat was using the limb normally.

Discussion
Excision arthroplasty is regarded as a salvage procedure to aleviate pain and restore a reasonable motility of the limb. Cats and small dogs, due to their light weight, seem to do well after the procedure and most have a wide range of movement of the limb. Large dogs (over 25 kg) can have problems with this treatment and other types of repair should be considered.

Complications that can occur from this procedure include damage to the sciatic nerve, resulting in possible paralysis, or chronic post-operative pain due to sharp edges being left on the femoral neck. There are several approaches to the hip joint, this one being considered to have the best exposure of the femoral neck. Post-operative care includes use of the limb, on a restrictive basis and active animals tend to have a quicker recovery than obese lethargic animals, as do animals that have suffered acute trauma (as in this case) rather than chronic conditions. It is particularly important to maintain the highest possible aseptic technique during any orthopaedic surgery. The drapes for this procedure did not quite cover the whole cat, and drapes of adequate size to cover the whole animal should always be provided, in order that the surgeon can give total concentration to the procedure without having to think about any risk of contamination. Ideally, a barrier drape should be used in order to avoid any bacterial strike through occurring when drapes get wet due to blood loss or flushing. On this occasion a drape, wrapped round and secured with a towel clip, was used to cover the distal limb. When manipulation of the limb during surgery is required a sterile bandage wrapped round and tied is probably less likely to slip, allowing contamination of the surgeon. Pethidine, an opiate, was chosen for the analgesic as the procedure was a painful one and inclusion in the premedication ensured that it was active at the time of the first incision. Carprofen, a non steroidal anti-inflammatory drug, was given after induction to ensure it was active during recovery which is very important when using thiopentone which has an antanalgesic effect during recovery. Pethidine is a short acting opioid, about 2 hrs in the cat, and analgesia was repeated after 2 hours using 0.8 mg of morphine by intra-muscular route. Morphine is a more potent analgesic than pethidine and also remains active for longer, ie 6 - 8 hours in the cat, so could have been used initially in the premedication instead of pethidine. However, analgesia was satisfactory for this procedure and the cat was comfortable post-operatively.

With regard to the use of suture materials, being a publicly funded clinic, cost will always play an important part in choice between any two suitable suture materials. Chromic catgut often causes an inflammatory tissue reaction and polyglactin 910 (Vicryl) could have been used as an alternative suture material. When catgut is used it should be rinsed in sterile saline after removal from the package, as the solution it is stored in is an irritant to tissues. Polyamide is the standard material used for skin closure at this practice, but monofilament nylon or Vicryl could have been used. Hartmanns was a good choice of fluids for general maintenance under anaesthesia. Plasma expanding fluids might have been necessary had there been haemorrhage during surgery. The owners were told to put an elizabethan collar on the cat to stop removal of sutures. However, generally speaking if an animal removes sutures it is usually because they are irritant, ie too tight, or underlying catgut is causing a tissue reaction. The sutures in this case were placed quite loosely to allow for any swelling of tissues and the owners stated that they found it unnecessary to use a collar. 

References:
Atlas of Surg Approaches to Bones of Cat & Dog, Piermattei, Greeley. 2nd Ed Saunders 1979
Sm An Orthopaedics, Brinker, Piermattei, Flo. 2nd Ed. Saunders 1990
Sm An Surg Nursing, D Tracy, 2nd Ed. Mosby 1994