Complications Associated to TPO (Triple Pelvic Osteotomy)
Aldo Vezzoni, MedVet, Dipl ECVS,
Clinica Veterinaria, via Massarotti
Triple Pelvic Osteotomy (TPO) is a common surgical procedure for treatment of developing hip dysplasia in growing dogs. Since its introduction in veterinary field by Hohn and Janes in 1969, several surgical techniques has been described, but the one currently most used is the Slocum's technique, fixing the rotated ilium with the Slocum's Canine Pelvic Osteotomy Plate (CPOP) or similar pre-angled plates.
TPO is recommended in immature dogs with incipient hip dysplasia, with or without clinical signs but with no or only mild degenerative joint changes. Incipient hip dysplasia is evidenced by hip joint incongruity and subluxation of the femoral head with increased inclination of the dorsal acetabular rim (DAR) and joint laxity, with positive Ortolani sign showing a subluxation angle over 10° and a difference between reduction angle and subluxation angle of at least 10°. For TPO being effective, the DAR should be preserved, as viewed in the radiographic DAR view, to provide a potential dorsal acetabular support for the femoral head. Most of the potential candidates for TPO surgery are between 5 and end 10 months of age, with better indication in the youngest ones, where bone healing is faster, body weight is lighter, and bone plastic adaptation to the new relationship between acetabulum and femoral head is still possible. Rigid internal fixation of the ilial osteotomy is responsible for stability of the acetabulum after its lateral rotation promoting bone healing of the ilium and ischium osteotomies. The prognosis after TPO is excellent, provided that suitable indications were followed, proper surgical technique was accomplished and good postoperative care was guaranteed.
Complications in TPO
TPO has several specific complications that are most commonly related to wrong surgical indications, to mistakes in surgical technique and to poor postoperative care; they consist in progressive degenerative joint disease (DJD), persistent joint incongruity, reduced abduction and abnormal gait, nerve injury, intraoperative severe bleeding, excessive narrowing of the pelvic canal and implants failure.
To completely avoid any degenerative joint disease, TPO should be performed only with proper indications and when no degenerative sign is already present. Performing TPO in dogs with loss of DAR that looks abraded and rounded, acetabular filling and cartilage damage, when degenerative joint changes with osteophytes and bony changes were already established, will not avoid the progression of severe arthrosis. Any conditions requiring a rotation of the acetabulum more than 20° will anticipate some degree of arthrosis, because of the advanced underlying dysplasia. TPO performed in adult dogs and in young dogs close to skeletal maturity will not allow the bone remodelling of acetabulum and femoral head required to achieve a good joint congruity; as a consequence the new joint loading will not be well distributed on all the joint surface, causing cartilage degeneration and arthrosis. The decision to perform TPO in dogs that are out of a proper indication should take in consideration the risk/benefits balance, anticipating a limited progressive djd that should any way improve the original condition. Performing bilateral TPO in two different surgical times will cause a worsening of the second treated hip, that could be avoided by a contemporary treatment.
Persistent joint incongruity
Performing TPO in dogs with excessive joint laxity and failure, without addressing it with additional surgical procedures, could not avoid persistent subluxation of the femoral head and damage to the DAR. After surgery Ortolani sign should be negative or minimal; a still positive Ortolani sign is anticipating persistent joint subluxation and degeneration.
Reduced abduction and abnormal gait
Rotating the acetabulum more than 20° could cause an impingement of the DAR on the femoral neck, limiting leg abduction and damaging the dorsolateral border of the acetabulum; 30° rotation will usually limit hip extension of some degree and will cause sometimes an abnormal gait with the leg carried laterally when advanced. Dogs with limited hip extension can show hyperextension of the hock.
TPO is an invasive surgical technique and neurological deficits following the procedure are possible, mainly involving the ischiatic, pudendal and obturator nerves. Physical injury of peripheral nerves may result from compression, traction, laceration or contusion. Prognosis depends on the type of damage and degree of dysfunction. In dogs with complete loss of motor and sensory function prognosis is usually poor. If the nerve sheath is intact prognosis is better but reinnervation must usually occur in a maximum of one year to be effective. EMG, nerve conduction studies and evoked potential findings have a great value in establishing a prognosis and in assessing the progression of recovery.
The obturator nerve may be damaged performing pubic osteotomy and should be protected with a blunt periosteal elevator; its injury could cause lateral sliding of the limb while standing on smooth surfaces because of atrophy of internal muscles of the tight. The cranial gluteal nerve may be damaged exposing the ilium, particularly the branch supplying the tensor fascia lata muscle, crossing the surgical filed; its injury seems not be clinically relevant. The ischiatic nerve could be injured by the tip of an Hohmann retractor seated behind the greater ischiatic notch pinching the nerve, or by the ilium osteotomy procedure (osteotome, oscillating saw, reciprocating saw) or by drilling the holes in the ilium for placement of the plate-screws.
Ischiatic nerve protection should be accomplished avoiding Hohmann retractors placed on the greeter ischiatic notch and by using a Langenbeck periosteal elevator positioned flat to the bone taking care not to pinch the nerve, by protecting the nerve during the osteotomy with sponges inserted between the ilium and the nerve, or with a spoon or a malleable retractor in the same position, or by using an oscillating saw or an osteotome with a predetermined stopping device. Great care should be used when drilling the holes for plate fixation to avoid the underlying nerves and the vessels; the holes in the acetabular part of the ilium should be drilled lifting and pulling cranially the bone so it will overly to the proximal ilium segment, and the holes in the proximal part of the ilium should be done protecting the nerves with a spoon or a malleable retractor or with a finger gently pushing the nerve away from the drilling direction. Further protection of soft tissue when drilling could be achieved by using an oscillating device mounted on the drilling machine to avoid soft tissue twisting. Ischiatic nerve injury causes a severe limb paresis because only the extensor muscles of the stifle are spared. The dog may support its weight but the stifle doesn't flex. Injuries involving the ischiatic components of the lumbosacral trunk also disrupt the tibial and peroneal nerve fibers, thus the hock and the digits can neither flex nor extend. The dog stands on his knuckles and the hock is passively extended. Sensory function is compromised distally to the stifle.
Injuries to the pudendal nerve could be determined by an improper too proximal iliac osteotomy, involving the S1 root, or by a careless procedure, or by careless and improper (too dorsal) drilling into the sacrum (drilling should always be perpendicular to the plate, avoiding any ventro-dorsal inclination which could be directed to the sacral foramens and to the sacral neural canal); their damage will cause urinary and fecal incontinence due to the loss of tone of the sphincters and of the perineal muscles and sensory impairment of the corresponding dermatomes. Pelvic nerve injuries could be caused by the same procedures that would injury the pudendal nerve and are responsible for the disorders of micturition caused by bladder paresis or paralysis associated with urine retention. Injuries to the distal part of the pudendal and perineal nerves could be caused by an excessively wide approach to the ischium extending too medially and will cause urinary and fecal incontinence due to the loss of tone of the sphincters and of the perineal muscles and sensory impairment of the corresponding dermatomes.
Intraoperative severe bleeding
Great care should be used when drilling the holes for plate fixation to avoid the nerves and the vessels medial to the ilium, particularly the internal iliac artery and vein which lie under the lumbosacral trunk. Injures to this vessels has been reported causing fatal hemorrhage. Drilling holes in the distal ilium should be performed while keeping it lifted cranially over the proximal ilium.
Excessive narrowing of the pelvic canal
Bilateral TPO could cause a 20% reduction of the pelvic width, due to rotation of the pelvis with the procedure; the reduction could be higher if associated to implant failure, to higher rotation degrees and to the thickness of the pubis ramus left medial to the acetabulum; while this narrowing would inhibit delivery in a pregnant beech, requiring cesarean section, reproduction of TPO treated dogs is routinely discouraged. Excessive narrowing of the pelvic canal causing fecal constipation is very rare.
Implant failure is the most common reported complication following TPO; in a personal study performed on 218 TPOs in 162 dogs, implant failure occurred in 49, representing the 25% of them all. Loosening of one or more screws was observed in 86% of all implants failure while rupture of one or more screws in 14%. Loosening of the screws was more frequent with cortical screws and rupture of the screws was more associated to cancellous screws. Implant failure was more frequent in the proximal part of the plate (72%) than in the distal one (26%), and rarely in both (2%). Occurrence of implant failure occurred more frequently in the left hips (34%) than in the right hips (12%), possibly associated to the direction of cycling motion. Cerclage wire on the plate and cerclage wiring of the ischium osteotomy was associated to a significant reduction in implant failure.
While implant failure is a frequent complication, surgical revision is usually not required as its influence on bone healing and on joint congruity is not deleterious; this may be due to the relative stability provided by the remaining fixed screws, to the good muscular support of the osteotomy, to the early callous formation in young dogs and to the direction of the forces into the hip maintaining joint congruity.
Implant failure may be related to several factors, including poor postoperative care, early weight bearing in bilateral cases, weak holding power of screws in juvenile bone, hyperactive nature of young dogs, body weight and, for a lesser degree, poor surgical technique and material failure. In our study, the hyperactive nature of the dog and the body weight represented the main predisposing factor to implant failure, being more frequent in male dogs (70%) than in female dogs (30%); breed occurrence was more frequent in German Shepherds (52%) and in Rottweilers (32%), than in Retrievers (17%).
When TPO is performed bilaterally in the same time, cage rest and helping the dog walking with a support under the groin to lift it for 8-10 days is very useful to keep the dog comfortable and to limit the occurrence of implant failure.
In general, good postoperative care with good client compliance in the two months following surgery is essential to reduce the incidence of implant failure; this may be also lowered applying the cerclage wire on the distal part of the plate, the cerclage wire on the ischium osteotomy, cutting the distal insertion of the sacro-tuberous ligament, using cancellous screws in very young dogs and cortical screws in older ones, inserting the screws in a slightly divergent direction between them.
Complications in Triple Pelvic Osteotomy may be significantly lowered by proper case selection, good surgical technique and good postoperative care, giving to this procedure an important place in the early treatment of canine hip dysplasia.
Aldo Vezzoni, Med Vet, Dipl ECVS