Ross H. Palmer Department of Clinical Sciences, Colorado State University, Fort Collins, CO, USA Patellar luxation is a common cause of lameness in dogs and less commonly in cats and can be either medial or lateral. This chapter will focus on medial patellar luxation (MPL), as it is more common overall. Canine MPL is graded I‐IV according to severity.1 Healthy static and dynamic patellofemoral joint constraints support normal patellar tracking. Static constraints include normal patellofemoral joint conformation in which the patella glides through a deep femoral sulcus bounded by medial and lateral trochlear ridges throughout a full range of stifle joint motion. Static constraints also include balanced tension within the joint capsule, retinaculum, and medial/lateral femoropatellar ligaments. Dynamic constraint is provided by proper alignment of the quadriceps‐patellar mechanism (QPM) with the underlying skeleton and is necessary for normal patellar tracking. MPL may be congenital, developmental, or traumatic, but the majority are developmental in nature. While not fully understood, malalignment of the QPM with the underlying skeleton plays a key role in MPL development and condition severity, and it is key to understanding the relationship between MPL grade and the surgical treatments necessary. The patella is a sesamoid bone within the active quadriceps muscle group. Tensile forces within the quadriceps group may either pull the patella firmly into the femoral trochlear sulcus or generate forces favoring medial or lateral patellar luxation, depending upon the skeletal malalignment relative to the overlying QPM (Figure 49.1). When QPM forces favoring patellar luxation are relatively small, the static constraints of the patellofemoral joint are able to maintain normal patellar tracking. When these static constraints are incapable of overcoming strong active QPM forces, spontaneous patellar luxation occurs. There is a profound difference between lesser MPL grades (I–II) and more severe grades (III–IV) upon skeletal development and soft tissue distortions. Sustained displacement of QPM relative to the skeleton (i.e., grade III–IV patellar luxation) during skeletal development can lead to anatomical changes in the femur and tibia. The absence of sustained normal retro‐patellar compression on the femur can lead to trochlear hypoplasia, as well as the development of a misplaced pseudo‐sulcus. The bowstring effect of sustained medial QPM displacement can contribute to coxa vara, femoral varus, and external torsion, as well as medialization of the tibial tuberosity and external tibial torsion.2–4 MPL causes internal rotation of the stifle joint and, when sustained, can lead to profound soft tissue distortions, such as medial capsular/fascial contracture, lateral capsular/fascial stretching, and likely alterations to meniscal shape/structure. Figure 49.1 Dynamic patellofemoral joint constraint. The patella is a sesamoid within the powerful quadriceps muscle group. When there is skeletal malalignment with the overlying quadriceps patellar mechanism (QPM), quadriceps tonus may exert a force favoring patellar luxation (medial in this example). Careful observation of gait is essential and must be correlated with limb palpation findings to fully assess the luxation grade and associated skeletal deformities that may result. In brief, lesser grades of patellar luxation (I–II) are much less likely to be associated with major soft tissue distortions and skeletal deformities, because the QPM is properly aligned with the underlying skeleton most of the time in these cases (i.e., the patella is reduced more often than it is luxated). In contrast, more severe grades of patellar luxation (III–IV; especially grade IV) are commonly associated with profound soft tissue distortions and skeletal deformities even in small breeds of dogs.2,3 Gait observation includes classifying the gait as normal or lame, grading the lameness, and describing any lameness as sustained versus episodic. The degree of lameness may vary according to gait: stance, walk, trot, run, and ascending versus descending stairs. Grade II MPL is often characterized by an episodic skipping gait in which the gait may be normal one moment and non‐weight‐bearing the next. It is also important to note that some dogs, especially terrier breeds, can display such a skipping gait but do not have patellar luxation. Gait observation also includes noting pelvic limb conformation. Dogs with MPL often appear to be “bowlegged” (i.e., genu varum), and this may be a true skeletal deformity or may merely be a postural change that resolves when the patella is reduced.5 Finally, it is important to note the foot position during stance and correlate it with the luxation grade; for instance, internal rotation of the foot (i.e., “pigeon‐toed posture”) with grade III luxation is likely the result of internal stifle rotation. In contrast, the normal orientation of the foot in association with grade IV MPL is strongly suggestive of an external tibial torsional deformity; this is because grade IV MPL means the stifle (proximal tibia relative to the femur) is internally rotated, yet the foot remains in the normal orientation rather than being aligned with the proximal tibia. When lameness is detected, it is important to correlate the patellar position (normal versus medial versus lateral). The direction of spontaneous patellar luxation must be determined before performing surgical correction. It may be preferable to palpate the patellar position with the patient in a standing position for several reasons: (1) it can be correlated with the gait that was observed immediately prior, (2) stance keeps the quadriceps active and, therefore, more reflective of the dynamic QPM, and (3) it is relatively easy to simultaneously extend the hip such that the rectus femoris is placed under tension. (Note: the rectus femoris is the only of the four quadriceps muscles to span the hip as well as the stifle joint). Standing exam may also be helpful in detecting joint effusion / periarticular fibrosis that is particularly common when there is concomitant cranial cruciate ligament disease (CrCLD). If the patella is difficult to palpate, it can be helpful to follow the tibial tuberosity and patellar ligament proximally. Once detected, the patella is held between the thumb and forefinger as the hip is extended and the stifle joint is rotated internally (to assess for MPL) and externally (to assess for LPL). When luxation is detected, it should be graded, and any patellofemoral crepitus is noted, which is an indicator of cartilage erosion of the patella and/or trochlear ridge. While tibial tuberosity position may be noted, it is ideal to reduce the patella before doing so, because medial luxation rotates the entire tibia internally such that apparent medialization of the tibial tuberosity may be postural rather than anatomic. To that end, in most instances, final decisions about the patient need for tibial tuberosity transposition (and trochleoplasty) are typically made intra‐operatively, as will be discussed. It is important to evaluate for cranial drawer and/or cranial tibial thrust instability, as concurrent treatment of MPL and CrCLD, while advisable, is a more complex surgical procedure that is associated with higher rates of surgical complication; referral to a surgical specialist may be advised. Stifle radiographs help to confirm luxation (though grade II MPL may appear normal), detect degenerative changes, and identify occult co‐morbidities. Radiographs of the femur and tibia are often required to detect complicating skeletal deformities, but caution should be exercised because subtle shifts of a three‐dimensional object (such as a bone) can create the artifactual appearance of a deformity on a two‐dimensional radiographic image.6,7 Mediolateral (M‐L) positioning is often simpler than latero‐medial position when seeking to obtain a sagittal plane image of the stifle, femur, or tibia. An M‐L stifle view is helpful for detecting joint effusion and caudal superimposition of the patella upon the femoral condyle, which may suggest patellar luxation. Caudo‐cranial (Cd‐Cr) position is often simpler than the cranio‐caudal position when seeking to obtain a frontal plane image of the stifle, femur, or tibia. The direction of patellar luxation, when present, can be detected on properly positioned Cd‐Cr views. Properly positioned sagittal‐ and frontal‐plane radiographs can be used as a screening tool to rule out significant skeletal deformities, but computed tomography (CT) is advised to definitively diagnose and quantify deformity.7 Failure to detect and treat skeletal deformities contributing to grade III and IV MPL is a common cause of postoperative re‐luxation. It must be noted that patellar luxation corrective surgery is “part art and part science”; as such, the following is reflective of that current reality. Surgery is generally indicated for those cases of symptomatic MPL, particularly if the symptoms are progressing in frequency and/or severity. Grade II and some early/mild grade III MPLs can typically be managed by some combination of trochleoplasty, tibial tuberosity transposition, and soft tissue tension balancing. The final decision regarding the necessity of these procedures is typically performed intra‐operatively. The necessity of and planning for femoral and tibial corrective osteotomies, most common with grade III and grade IV MPLs, is typically performed pre‐operatively from CT images and emphasizes the importance of deformity detection and luxation grading through correlation of gait, stifle examination, and imaging. It is important to note that surgical treatment of MPL is not performed identically in each patient or even within defined grades of luxation; instead, it is important to make pre‐ and intra‐operative assessments to identify the factors contributing to luxation in the individual patient such that surgical treatment can be customized accordingly. Bony contributors to MPL, such as trochlear hypoplasia and medial displacement of the tibial tuberosity, cannot be corrected with soft tissue treatments, and soft tissue contributors to MPL, such as medial joint capsular contracture and lateral capsular stretching, cannot be corrected with bony treatments. The typical sequence of surgical assessments and treatments is as follows: The patient is typically positioned in dorsal recumbency with the pelvic limbs toward the end of the table and, when the hip is abducted, the tibia can lie flat with its lateral surface on the surgical table and oriented in the perfect M‐L orientation (sagittal plane). Once the proper patient position on the table is confirmed, the limb is suspended per usual so that an aseptic hanging limb preparation and draping can be performed. The need for trochleoplasty is assessed via the 50/50 rule (Figure 49.2). This is a subjective visual assessment in which, following arthrotomy, the patellofemoral joint is visualized from full stifle joint extension to full flexion while asking the question, “Is 50% of the depth and length of the patella contained within the trochlear sulcus?”. If the answer is YES, the patient does not need a trochleoplasty. If the answer is NO, trochleoplasty is indicated, and the dimensions of which should satisfy the 50/50 rule. For instance, it is relatively common to discover “relative patella alta” in which only the distal 30%–40% of the patella is within the trochlear sulcus during full stifle extension. In this instance, the trochleoplasty is extended proximal to the native trochlear sulcus, such that the 50/50 rule is satisfied. Often erosion of the proximal extent of the medial trochlear ridge (and mirrored erosion of the disto‐lateral patellar articular surface) is noted to support this conclusion. One could certainly argue that the stifle is not typically held in full extension during the gait cycle, but a slight exaggeration of the proximal trochleoplasty dimension in an effort to reduce the reported incidence of postoperative re‐luxation is the rationale.8 As to the abaxial dimensions of the trochleoplasty, it is essential that the trochleoplasty be able to receive the full width of the patella; thus, these are typically “just within guardrails” represented by the peaks of the medial and lateral trochlear ridges. While making too narrow of a trochleoplasty is a common surgical error, it is also important that the trochleoplasty not be so wide as to excise the trochlear ridges. The dimensions of the selected trochleoplasty, per the 50/50 rule assessment, are typically outlined with a scalpel blade prior to performing either wedge or block trochleoplasty. Figure 49.2 Medial stifle arthrotomy (P = patella; F = femoral medial trochlear ridge; TT = tibial tuberosity). The 50/50 Rule is a subjective visual assessment used to determine if the patient requires a trochleoplasty and, if so, what the length and depth dimensions of the trochleoplasty should be. The patellofemoral joint is viewed through the full extension/flexion stifle range of motion to assess if at least 50% of its length and depth is always within the trochlear sulcus. Generally, techniques that preserve the healthy hyaline cartilage of the trochleoplasty are indicated. These include wedge recession, block recession, and semi‐cylindrical recession trochleoplasty techniques; while there are theoretic advantages and disadvantages of each of these relative to one another, each can be effectively used. Abrasion trochleoplasty removes the existing cartilage from the trochlear sulcus and is replaced by less congruent and resilient fibrocartilage. Abrasion trochleoplasty is, therefore, only indicated when preservation of healthy hyaline cartilage is not feasible (e.g., revision of previous abrasion trochleoplasty, intra‐operative complication with recession trochleoplasty, etc.).
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Medial Patellar Luxation Repair
Introduction
Etiopathogenesis of Medial Patellar Luxation
Gait Observation and Physical Examination
Diagnostic Imaging
Surgical Treatment
Trochleoplasty
Wedge Recession Trochleoplasty (WRT)
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