Medial patella luxation (MPL) is one of the most common stifle problems encountered in veterinary medicine. It is a more common orthopedic problem in the dog, but has been reported frequently in the cat. MPL may be caused by trauma or congenital malformations (with the latter being far more common) or may occur secondary to fracture disease and/or surgical intervention. Congenital MPL is most commonly observed in the toy and miniature breeds of dogs and also in the Abyssinian and Burmese cats and causes minimal, if any, to severe gait abnormalities. The degree of deformity and, therefore, subsequent dysfunction varies markedly within the affected population and within the approximately 50% of the patients with bilateral involvement. Many affected animals are presented at approximately six months of age with the owner describing a skipping or hopping type of gait or an animal that is intermittently non-weight bearing on one limb or the other. The problem is usually transient with the dog or the cat returning to apparently normal function within a short period of time. Although medial patella luxation may be present in cases with acute severe lameness, it is usually a chronic problem, and therefore, other causes of acute onset of stifle pain (i.e. cruciate ligament injury) should be carefully ruled out. Other animals are presented as adults with a history of acute onset of lameness with minimal trauma or with the frequently severe and complicated secondary stifle changes due to chronic abnormal stress. There are also a certain portion of the affected population that present for routine physical examination with severe grades of MPL, to which the owner responds, “What do you mean he has bad knees, he’s never limped a day in his life.”
Various etiologies have been suggested for congenital medial patella luxation; however, it is generally agreed that MPL is a multi-factorial disorder of pelvic limb conformation. Depending upon the severity of the individual case, various combinations and degrees of the following features may be observed: (1)intermittent or permanent medial patella luxation, (2)medial displacement of the quadricep’s group with or without muscular contraction, (3)limited extension of the hip and stifle, (4)decreased antiversion, (5)coxa vara, (6)external rotation and/or torsion of the distal femur, (7)internal rotational laxity of the tibia and/or medial torsion of proximal tibia and lateral torsion of the distal tibia, (8)medial displacement of the tibial tuberosity and crest, (9)real or artifactual lateral bowing of the distal femur with increased caudal and medial concavity, (10)lateral angulation or tilting of the femurotibial joint space, (11)shallow or absent trochlear groove, (12)femoral and tibial condylar asymmetry, (13)genu varum, (14)redundant or stretched lateral joint capsule, (15)contracted or inadequate medial joint capsule, (16)cartilage fibrillation or erosion on the articular surface of the patella, (17)erosion on the medial aspect of the dysplastic medial femoral condyle, (18)meniscal, cranial cruciate and lateral collateral ligament changes, and lastly, (19)degenerative joint disease.
While these derangements are common in cases of MPL in the dog, abnormal shape of the femur, tibia, or both has not been observed in cats with MPL. Some cats, however, do exhibit shallow trochlear grooves and medial deviation of the tibial tubercle. Because of the broad range of clinical lameness and degree of deformity present in this syndrome, no one surgical procedure is appropriate for all affected patients. Numerous surgical procedures have been described for the repair of medial patella luxation, and determining the most effective combination of procedures to utilize is quite a challenge. The goal of surgery is to realign the quadricep’s extensor apparatus and to, therefore, stabilize the patella in the trochlear groove and return the animal to at least good, if not excellent function. A grading system, based on the severity and types of deformity present provides the basis for a rational surgical approach. For each grade of malformation, the following discussion will present the surgical techniques and treatment rationale recommended for the treatment of MPL in the dog and cat at the current time.
Grade 1 MPL - The anatomy of the stifle joint is almost normal and the patella luxates only when the joint is extended and lateral digital pressure is applied; the patella reduces spontaneously. Because these patients have a normal to near normal stifle joint, minimal if any clinical signs are observed when the animal is presented. It is difficult to recommend a surgical intervention on a clinically normal animal based on the assumption that these animals may be prone to future abnormalities and injuries, and it is my opinion that they are not operated unless they become clinically symptomatic. In most, if not all, cases of Grade 1 medial patella luxation, all that is needed is the creation of a lateral restraint to prevent medial displacement of the patella. This is accomplished by either lateral imbrication of the joint capsule or placement of an antirotational suture around the lateral fabelle and through a tunnel drilled in the tibial tuberosity. In a small number of cases, a medial retinacular release may also be required.
Grade 2 MPL - The patella usually resides within the trochlear groove; however, it will luxate upon flexion of the joint (or digital pressure) and remains luxated until relocated by digital pressure or active flexion and extension of the joint. These animals usually present with some degree of gait abnormality because of the presence of some angular and torsional deformity, and degenerative changes are more likely to develop over time. In these cases, soft tissue procedures alone are usually not capable of restoration of function. A trochleoplasty, medial retinacular release, and lateral imbrication are usually sufficient to overcome the problem; however, if the tibial tuberosity is deviated medially, a tibial crest transplant is also indicated.
Grade 3 MPL - The patella is luxated most of the time, but may be reduced temporarily while the limb is extended. The angular rotational and torsional deformities are more severe than in Grade 2 MPL, and a corresponding increase in clinical signs of lameness are observed. All of the previously mentioned procedures will probably be necessary to correct Grade 3 MPL; however, the rotational instability of the tibia will still need to be addressed. Placement of an antirotational suture around the lateral fabelle and through a tunnel drilled in the tibial tuberosity is usually indicated to help correct a Grade 3 MPL.
Grade 4 MPL - The patella is luxated period, and no amount of manual pressure of full limb extension will result in reduction. Thankfully, this grade is the least commonly encountered, as the bony and soft tissue derangements are terribly severe and the previously mentioned techniques are inadequate to correct the problem. These cases usually require tibial and/or femoral corrective osteotomies to restore alignment of the stifle joint. In the majority of cases, only a derotational proximal tibial osteotomy is required to correct the malalignment of the distal tibia and paw, which results from derotation of the proximal tibia by imbrication suture. In some severe cases, however, a cuneiform osteotomy of the distal femur may be necessary to help further align the quadricep’s extensor apparatus. As mentioned previously, as Grade 4 MPL is not encountered frequently; these corrective osteotomies are not usually necessary and should be used reluctantly, only in severe cases, and in my opinion, as a salvage procedure. These procedures should not be performed if there is the presence of permanent interarticular change (i.e. meniscal remodeling and/or cruciate or collateral ligament changes) as surgical intervention in an attempt to return the animal to more normal function may, in fact, make it worse, which brings us to another important point, namely, timing of the surgical procedure in order to achieve the best functional results.
Once clinical lameness has been recognized, surgical intervention is warranted. In cases where severe bone distortion is not a component of the problem, surgery should be performed early to prevent the continued degeneration of the joint, which normally occurs, and to prevent excessive strain and stresses from developing on the cranial and caudal cruciate ligaments, which will result in their further deterioration. In those cases with evidence of bone distortion, early surgical intervention is a necessity, as the effects of continual growth distortion cannot be ignored. In regard to specific surgical technique, the aforementioned procedures are relatively straightforward with the exception of the corrective osteotomies. The only real choice to be made is how to perform the trochleoplasty. The older, more standard procedure requires excision of trochlear cartilage and some subchondral bone to achieve proper trochlear depth and maintenance of patella alignment. As hyaline cartilage does not regenerate, the exposed bone which results is fast covered by granulation tissue which undergoes metaplasia to fibrous tissue and finally fibrocartilage, which is frequently thin, incomplete and of variable quality. With the advent of the more advanced techniques currently available, this approach can no longer be recommended.
A second technique relies on elevation of the trochlear cartilage and removal of subchondral bone with subsequent replacement of the hyaline cartilage. This technique is applicable to dogs under 5 months of age, as cartilage elevation becomes more difficult as the animal ages, as the articular cartilage adheres more tightly to the underlying subchondral bone. In the trochlear recession wedge technique, the femoral trochlea is depressed while maintaining the normal hyaline cartilage surface, and has provided excellent results in dogs of all ages. An advantage of the recession wedge technique is that trochlear patella articular cartilage contact is maintained, accounting for a quicker return to full function of the limb.
Most recently, trochlear block recession (TBR) has been described in the veterinary literature as an alternative to trochlear wedge recession (TWR). In this technique, the surface of the recessed osteochondral block is rectangular, rather than a wedge which tapers to a point proximally and distally. The rectangular shape preserves a larger area of hyaline articular cartilage as well as a greater width to the trochlear recession proximally. Therefore, the value of this procedure is its ability to increase proximal pafuctellar depth, increase patellar articular contact with the recessed proximal trochlea, recess a larger percentage of the trochlear surface area, and to better resist patellar luxation in an extended position as compared with TWR.
The TBR was proposed as an alternative to the TWR as a method to increase recession of the proximal trochlea, which is important in the clinical treatment of patellar luxation. With the limb in a flexed position, there are no significant differences between TWR and TBR. As the patella most easily luxates when the stifle is extended and the patella is in a more proximal location within the trochea, performance of a TBR will increase proximal patellar depth and therefore increase patellar articular contact with the recessed proximal trochlea to better resist patellar luxation with the limb in an extended position as compared with TWR.
In conclusion, a variety of surgical techniques are available for correction of the various grades of medial patella luxation and a combination of procedures is usually necessary to optimize return to function. Once clinical lameness is observed, rapid surgical intervention should be considered to help prevent the often-severe debilitating soft tissue and orthopedic changes, which result in premature development of degenerative joint disease.
The utilization of medical therapy in conjunction with surgical intervention affords the best clinical response. Medical therapy should include but not be limited to the administration of pain management medications (opiods and opiod transdermal patches, non-steroidal anti-inflammatories), neutraceutical supplements (glucosamine, chondroitin, msm, etc), polysulfated glycosaminoglycans (Adequan), acupuncture, physical therapy, Class IV laser therapy, and platelet rich plasma administration. With proper medical and surgical intervention, the majority of animals with patella luxation tendencies can be returned to at least good if not excellent, normal function.
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