What is the degree of flexion of the knee for a tangential view of the patella?

12.2.1 Anatomy

The patella, the largest sesamoid bone in the body, articulates only with the patellar surface of the distal femur (patellar notch). The patella rides in the tendon of the quadriceps femoris—the largest muscle of the thigh and the primary extensor of the knee. The patella functions to protect the knee joint, to lengthen the lever arm of the quadriceps femoris, and to increase the area of contact between the patellar ligament and the femur.

The apex of the patella is the nonarticular tip of the bone. It points distally.

The lateral articular facet for the distal femur faces posteriorly and is the largest part of the large articular surface of the patella.

The medial articular facet for the distal femur faces posteriorly and is smaller than the lateral articular facet.

The base of the patella is the blunt, nonarticular proximal end of the bone, opposite the apex.

Movements of the patella

The patella is capable of several motions due primarily to its small articular surface (as compared to its associated femoral surface), its lack of congruence, and the several directions of tension available through the quadricep fibers. When the knee is fully extended, the patella is suspended in front of the femur with little or no contact of the articular surfaces. As the knee joint flexes, the patella is seated between the femoral condyles and slides down the femur (patellar flexion), ending in full flexion by presenting its articular surface superiorly (facing the distal end of the femur). During this course of sliding distally, it may experience medial and lateral patellar tilting (rotation about a vertical axis) (Fig. 13.18) the degree of which depends upon the shape of the femoral condyles, which it must conform to en route. The patella may also be shifted medially or laterally (primarily by quadriceps tension), thereby creating more drag on the corresponding articular facets. When the tibia is medially or laterally rotated, the patella may also exhibit medial and lateral rotation about an anterior/posterior axis, being pulled into rotation by the tibia via the patellar ligament (Fig. 13.19).

Levangie & Norkin (2005) note, ‘Failure of the patella to slide, tilt, rotate, or shift appropriately can lead to restrictions in knee joint ROM, to instability of the patellofemoral joint, or to pain caused by erosion of the patellofemoral articular surfaces’. Additionally, as the knee flexes and extends, the quadriceps pull the patella superiorly while the patellar tendon (ligament) pulls it inferiorly, which actually results in posterior compression force onto the femur. ‘During the stance phase of walking, when peak knee flexion is only approximately 20°, the patellofemoral compressive force is approximately 25% to 50% of body weight (Heino Breacher & Powers 2002). With greater knee flexion and greater quadriceps activity, as during running, patellofemoral compressive forces have been estimated to reach between five and six times body weight. (Flynn & Soutas-Little 1995)’

A major contributing force in pulling the patella out of its normal track, which thereby influences excessive pressures on particular aspects of the facet surfaces, is that of imbalanced pull of the quadriceps muscles. The effect of the alignment of the quadriceps and patellar ligament as they pull the patella across the femoral condyles can be assessed using a measurement called the Q-angle (the quadriceps angle, see Fig. 13.23). The angle, measured with the knee in extension or slightly flexed, is formed by the intersection of a line running from the ASIS to the mid-patella and a line connecting the tibial tuberosity to the mid-patella. An angle of 10° to 15° is considered normal. When the Q-angle is excessive (due to the pull of lateral forces), the vastus medialis oblique is responsible for horizontally aligning the patella and preventing lateral excursion. If this portion of the quadriceps is weak, or if hypertrophy of the vastus lateralis exists, especially in the presence of a high Q-angle, this will likely produce imbalanced patellar tracking as well as increased compressive forces, including those onto the lateral lip of the femoral sulcus (Levangie & Norkin 2005) (see p. 475 for further discussion).

Patella

The patella is the largest sesamoid bone in the body and an integral portion of the quadriceps apparatus. Fractures of the patella are most commonly the result of direct trauma to the stifle from jumping over a fixed object or a kick from another horse. Patellar fractures result in variable lameness, soft tissue swelling, and effusion, depending on the amount and configuration of the fracture. Horses with patellar fractures commonly hold the limb in partial flexion, resting the limb with the toe touching the ground, and walk with a careful but weight-bearing gait. Radiographic evaluation should include not only the craniocaudal, lateromedial, flexed lateromedial, and caudal 30-degree lateral craniomedial oblique views but also the cranioproximal craniodistal (skyline) view of the patella to thoroughly assess the fracture. Patellar fracture morphology is highly variable; however, sagittal fractures are most common and usually involve the medial aspect of the patella (see Figure 99-8). Fragmentation of the distal aspect of the patella is most commonly associated with medial patellar ligament desmotomy.

Techniques for partial patellectomy and internal fixation have been described; however, fractures located at the base of the patella commonly do not require surgical intervention (Figure 99-33).76-78 Surgery should be attempted only if there is complete disruption of the quadriceps apparatus (i.e., inability to extend the stifle), the fracture gap is greater than 5 mm on radiographs, there is obvious malalignment of an articular fragment, or the fragments can be palpated under the skin. One should allow the swelling to decrease before surgical intervention is attempted. Medical management of a patella fracture should include administration of nonsteroidal anti-inflammatory drugs for 2 weeks and stall confinement for 2 to 3 months.

When there is disruption of the quadriceps apparatus or a very large displaced articular fragment, internal fixation is indicated (Figure 99-34). Screw fixation using lag technique with 5.5 mm cortex screws and/or application of small DCP or LCP plates may be performed via arthrotomy or arthroscopy of the FPT. Partial patellectomy is indicated in sagittal medial articular fractures.79,80 This can be carried out by removing up to one third of the patella arthroscopically. Portal placement should be about one third to one half of the distance between the distal aspect of the patella and the proximal tibia; portal placement too close to the patella makes it difficult to view fractures on its distal aspect. A mechanical resector and an arthroscopic scalpel are essential tools to separate the fragment from the parent bone. With an osteotome, the fragment is divided into smaller fragments, which subsequently are removed with Ferris-Smith rongeurs. Although it is undesirable, it may be necessary to perform an arthrotomy, to dissect the fragment from its extensive patellar ligament attachments. Concurrent direct trauma to the lateral trochlear ridge is common, and should be evaluated and débrided arthroscopically. The prognosis after partial patellectomy for sagittal transverse fractures is favorable, with reports of an 83% to 100% return to full athletic function in cases with no pre-existing stifle arthritic changes.80,81 When the quadriceps apparatus has been disrupted, the prognosis is guarded to unfavorable, depending on the success of the repair.

PATELLA

The patella is the largest sesamoid bone in the body and an integral portion of the quadriceps apparatus. Fractures of the patella are occasionally diagnosed and usually result from some direct trauma. These fractures cause variable lameness, soft tissue swelling, and effusion depending on the amount and configuration of the fracture. Radiographic evaluation should include not only the craniocaudal, lateromedial, flexed lateromedial, and caudal 30-degree lateral craniomedial oblique views but also the cranioproximal craniodistal (skyline) view of the patella to make a complete assessment of the fracture. Patellar fracture morphology is highly variable; however, sagittal fractures are most common and usually involve the medial aspect of the patella (see Fig. 101-8).

Techniques for partial patellectomy and internal fixation have been described; however, fractures located at the base of the patella are rarely emergencies and do not normally require surgical intervention35,47,48 (Fig. 101-30). Surgery should be attempted only if the fracture gap is greater than 5 mm on radiographs, there is obvious malalignment of an articular fragment, or fragments can be palpated under the skin. One should allow for the swelling to decrease before surgical intervention is attempted. Medical management should include administration of nonsteroidal anti-inflammatory drugs for 2 weeks and stall confinement for 2 to 3 months.

Only when there is disruption of the quadriceps apparatus or a very large displaced articular fragment is internal fixation indicated (Fig. 101-31). Repairs have been performed through arthrotomy into the femoropatellar joint, and lag screw fixation using two or three 5.5-mm cortex screws placed from the apex to the base has been described. Partial patellectomy is indicated in sagittal medial articular fractures.49,50 This can be carried out by removing up to one third of the patella through arthroscopy. A mechanical resector is used on the fragment to create a separation between the fragment and the parent portion of the bone. With an osteotome, the fragment is divided into smaller fragments, which subsequently are removed with Ferris-Smith rongeurs. However, it is often necessary to perform an arthrotomy, because dissection of the fragment from its extensive patellar ligament attachment can be difficult using only arthroscopic techniques. The prognosis after partial patellectomy for sagittal transverse fractures is favorable. When the quadriceps apparatus has been disrupted, the prognosis is guarded to unfavorable, depending on the success of the repair.

Stifle

The patella is a sesamoid bone lying in the tendon of the stifle that inserts on the tibial crest. When the leg is extended (with relaxed quadriceps muscle) the patella can be displaced only very slightly medially and laterally, this movement being limited laterally by the retinaculum that passes from the patella to the fabella and by the joint capsule medial and lateral to the patella (Fig. 17.19). The fabellae, which are the sesamoid bones of the heads of the gastrocnemius muscle, are located on the caudal side, lateral and medial to the femoral condyles at the height of the patella.

The cranial cruciate ligament passes ‘like a hand in the pants pocket’ (from caudolateral to craniomedial), and the posterior cruciate ligament crosses it (Fig. 17.19). The cranial cruciate ligament prevents forward displacement of the tibia in relation to the femur and also limits endorotation of the tibia. The caudal cruciate ligament prevents caudal displacement of the tibia. The lateral collateral band, which passes from the femur to the fibula, prevents adduction of the tibia in relation to the femur. The medial collateral ligament passes from the tibia to the femur and prevents abduction of the tibia. Hyperflexion, hyperextension, endorotation, and exorotation should not be painful. The menisci give relief to the tibial plateau and function as shock absorbers. The medial meniscus is firmly attached to the medial collateral ligament.

In the examination of the right stifle the right hand is used to grasp the distal tibia and the left hand is placed over the stifle joint.1,2 The stifle is then extended and flexed and note is taken of the range of motion, crepitation, signs of pain, and the possible occurrence of a snapping sound. Local thickening, crepitation, and painfulness between the patella and the lateral fabella, just lateral to the edge of the trochlea, may indicate avulsion of the insertion of the long digital extensor muscle. Finally, the stifle is fully flexed (hyperflexion) and extended (hyperextension).

Next the examiner places the right hand around the metatarsus with the thumb medial to the calcaneus. The left thumb is placed on the lateral edge of the patella of the extended stifle. While the right hand exorotates the calcaneus and thereby endorotates the tibia in relation to the femur, the left thumb presses the patella in the medial direction (Fig. 17.20). The patella should remain in the trochlea and the endorotation should cause no evidence of pain. Then the thumb of the right hand is placed lateral to the calcaneus and the forefinger of the left hand is hooked around medially behind the patella. While the right hand exorotates the tibia in relation to the femur, the forefinger pulls on the patella while the stifle is still extended (Fig. 17.20). In healthy animals the patella remains in the trochlea and there is no sign of pain. If the patella is luxated, the depth of the trochlea is determined.

Then the examiner checks whether the cranial cruciate ligament is intact. There are two manual maneuvers for this, both of which are based on checking the forward moveability of the tibia in relation to the femur: (1) the drawer movement and (2) the tibial compression test. For the first of these, the examiner places the left forefinger on the patella, the left thumb behind the lateral fabella, the right forefinger on the tibial crest, and the right thumb behind the head of the fibula. With the stifle extended, half flexed (45°), and then flexed, the examiner pushes the right thumb forward in the direction of the right forefinger; the left hand is not moved and serves as the reference point (Fig. 17.21). This movement is performed repeatedly, rapidly, and with appropriate strength. During this process the stifle is neither extended nor flexed and the tibia is not rotated, but rather an attempt is made to move the tibia forward parallel to itself. Sometimes the tibia is found to be permanently displaced forward and it must first be moved caudally and then again cranially in order to produce the drawer movement. The hand grip is such that only bony structures are grasped, so that a displacement of the right hand in relation to the left must represent a displacement of the tibia in relation to the femur. Attention is given to evidence of pain and to forward displacement and/or endorotation of the tibia in relation to the femur.

Following this, with the stifle extended, half-flexed and then flexed, an attempt is made to displace the right forefinger in the direction of the right thumb, to check the posterior cruciate ligament. Attention is given to evidence of pain and to caudal displacement of the tibia in relation to the femur. Testing for the drawer movement can be difficult in strongly muscled dogs which resist the examination and in such cases must be repeated under sedation or anesthesia.

A second test for damage to the cranial cruciate ligament is the tibial compression test (TCT).11 If the hock is bent while the stifle is extended, the anterior cruciate ligament prevents cranial displacement of the tibia. The metatarsus is grasped from below by the right hand and the forefinger of the left hand is placed over the patella, patellar ligament, and proximal end of the tibial crest (Fig. 17.22). This forefinger should detect no forward movement of the tibial crest if the hock is bent while the stifle is kept extended.

To check the collateral ligaments, the stifle is held almost fully extended (15°). The thumb of the left hand is placed on the lateral collateral ligament (Fig. 17.19), while the fingers of this hand support the stifle. The right hand is used to grasp the middle of the tibia and to adduct the tibia in relation to the femur (Fig. 17.23). This should not cause any widening of the lateral side of the joint space. To check the medial collateral ligament the forefinger of the left hand is placed on the maximal medial protrusion of the tibial plateau. While the right hand, still in the same position, abducts the tibia, the left forefinger is used to feel whether there is displacement of the tibia and widening of the joint space (Fig. 17.24a).

Finally, the medial meniscus is checked for damage by pressing on it strongly with the forefinger directly caudal to the medial collateral ligament (Fig. 17.24b), taking note of any swelling and evidence of pain. A snapping sound as a result of contact between the femur and tibia in certain meniscal lesions can have been observed earlier in the examination (during extension and flexion).

The Patella

Patella

The patella is the largest sesamoid bone in the body and an integral portion of the quadriceps apparatus. Fractures of the patella are most commonly the result of direct trauma to the stifle from unsuccessfully jumping a fence or a kick from another horse. Parasagittal fractures involving the proximal medial aspect of the patella are most common in horses failing to clear a jump, presumably because the patella is struck, pushed upwards, and the medial femoropatellar ligament avulses the fragment (see Figure 101-9).91,92 In many cases, it appears that the initial impact with the object occurs at the distal femur and subsequently projects on to the patella. This can be seen arthroscopically as crushing injuries on the distal trochlear ridges. Lesions on the trochlear ridges may develop over a period of weeks following a patella fracture, and accordingly may not be evident on the initial radiographs. Transverse and comminuted fractures have also been reported.93,94 Patellar fractures result in variable lameness, soft tissue swelling, and effusion, depending on the type and configuration of the fracture. Horses with patellar fractures commonly hold the limb in partial flexion, resting the limb with the toe touching the ground, and walk with a careful but weight-bearing gait (Figure 101-32). Radiographic evaluation should include a caudocranial, lateromedial, flexed lateromedial, a caudal 30-degree lateral craniomedial oblique view, as well as a cranioproximal-craniodistal (skyline) view of the patella to thoroughly assess the fracture. The latter should be considered an essential radiographic view in any horse with a suspected patellar injury (Figure 101-33). Fragmentation of the distal aspect of the patella is most commonly associated with medial patellar ligament desmotomy, but it can also be seen in association with untreated upward fixation of the patella.95–97 A syndrome of concurrent lateral collateral ligament avulsion and patella fracture has also been documented.98

Nondisplaced, nonarticular patellar fractures are usually managed conservatively or with simple débridement through a traumatic wound. Fractures involving the proximal portion of the patella also can be managed conservatively, especially in young horses (Figure 101-34).94,97,99 Stall rest with a progressive hand walking program over 3 to 4 months is recommended. Surgical intervention is recommended in fractures where a radiographically displaced fracture gap of more than 5 mm is seen, malalignment of the articular fragments is diagnosed, or in cases of disruption of the quadriceps apparatus (i.e., inability to extend the stifle). Partial patellectomy via arthroscopic or arthrotomy approaches, and internal fixation have been described.91,92,94,100 It is very important to allow the swelling to decrease before surgical intervention is attempted.

Partial patellectomy is indicated in parasagittal medial articular fractures that involve less than one-third of the patella.91,101 Direct arthrotomy102 and arthroscopy91 have both been described for partial patellectomy; arthroscopy is preferred because incisional complications are more likely to develop with larger arthrotomy incisions. A number of arthroscopic portals may be used, but a craniomedial arthroscopic portal halfway between the distal patella and the tibia and between the medial and middle patellar ligaments is frequently used. Portal placement too close to the patella can make it difficult to view the distal aspect of the fractures and to pass the arthroscope under the patella. The instrument portal(s) is (are) selected after evaluating the joint surfaces. If a proximal (suprapatellar) portal is needed, a spinal needle can be passed into the suprapatellar pouch to determine a suitable location for a proximal portal. It is ideal to maintain distention and direct observation of the pouch while either a second arthroscopic cannula or a switching stick is inserted if the scope needs to be placed in the suprapatellar pouch. Regardless of the exact location of the fracture, a mechanical resector and an arthroscopic scalpel are essential tools to separate the fragment from the parent bone. The basic technique involves the identification of the margins of the patellar fragment, transection the soft tissue attachments as close as possible to the fragment with a scalpel, followed by the use of a resector to further remove the remaining attachments until the fragments are loose enough to remove either intact or in pieces. With very large fragments (Figure 101-35), an osteotome and mallet can be used to divide the fragment into smaller portions that subsequently are removed with Ferris-Smith rongeurs. It may be necessary (and faster) to perform an arthrotomy to dissect the fragment from extensive patellar ligament attachments but closure of the arthrotomy incision must be performed meticulously. Concurrent direct trauma to a femoral trochlear ridge is common, and should be evaluated, and if necessary, débrided arthroscopically. The prognosis after partial patellectomy for parasagittal transverse fractures is favorable, with reports of an 83% to 100% return to full athletic function in cases with no preexisting arthritic stifle changes.91,102

Internal fixation should be considered in parasagittal fractures not amenable to partial patellectomy and sagittal fractures. Although some major patellar fractures can be repaired through a parapatellar arthrotomy and broad exposure of the femoropatellar joint, healing of that incision is associated with complications in larger horses (Figure 101-36). In most cases, the fracture is more easily reduced and repaired with a direct incision over the major fracture plane previously identified with needles and radiographs. Surgery can be performed in dorsal or lateral recumbency, provided the stifle can be maintained in extension. Large pointed reduction forceps are used to achieve and maintain fracture alignment as standard lag screw technique is used. Stab incisions away from the primary incision are usually needed for drilling, tapping, and screw placement. Intraoperative imaging must be used to assess reduction and screw placement. Although screw fixation in lag technique with 4.5-mm cortex screws has been described, 5.5-mm screws are usually a better choice because of their far greater bending strength (Figure 101-37).92 Washers should definitely be used with 5.5-mm screws because the screw head (same size as a 4.5-mm screw) can readily sink through the thin cortical shell and into the larger glide hole (Figure 101-38). An aiming device can be helpful for accurate screw placement. The incision should be closed with tension-relieving sutures with or without stents and covered for recovery. Specialized recovery systems such as a pool or sling are highly recommended following internal fixation of patellar fractures. Incisional dehiscence and infection rarely occur postoperatively. Maintaining the horse in a sling or cross-ties has been recommended to protect the implants and incision in the immediate postoperative period.92

Disruption of the quadriceps apparatus most commonly occurs with transverse fractures (Figure 101-39). These fractures require screw fixation in lag technique with tension-band wiring using 1.25-mm diameter wire or cranially placed small dynamic compression plates (DCPs) or locking compression plates (LCPs) (Figure 101-40). When the quadriceps apparatus has been disrupted, the prognosis is guarded to unfavorable, depending on the success of the repair.

Patella Fracture Repair

1.

Expose the patella via the lateral approach, as previously described. Identify the patella by rotating the distal quadriceps muscle.

Pass two K-wires lengthwise through the patella across the fracture line.

Rotate the quadriceps muscle back to its normal position and loop a figure-eight orthopedic wire around the pin ends on the cranial surface of the patella.

Twist both long strands of the figure-eight wire to tighten the tension band apparatus and close the fracture line of the patella.

Cut the pin ends as short as possible. Cut the twisted wire, leaving two twists.

Patella Luxation

Animals with congenital patella luxation are usually brought to veterinarians shortly after birth because they tend to crouch on the rear legs when attempting to stand. The patella luxation functionally disrupts the quadriceps apparatus, rendering the animal unable to hold the stifle in extension. The primary differential diagnosis that must be ruled out with this presentation is femoral nerve injury, which also causes failure of the quadriceps apparatus because of lack of strength in the quadriceps muscle, producing the same abnormal stance. Femoral nerve injury is more commonly seen in calves after dystocia than it is in small ruminants. A diagnosis of patella luxation is easily made by palpating the patella; a luxated patella easily dislocates either medially or laterally. In severely affected animals the patella remains luxated and is difficult to reduce into its normal position. This manipulation is more easily accomplished with the stifle held in extension.

Standard radiographic views with the addition of a skyline image demonstrate the position of the patella, the depth of the trochlear groove, and other osseous abnormalities that may be present. The skyline view, which allows the best assessment of the trochlear groove, is taken with the stifle flexed and the x-ray beam directed proximally to distally perpendicular to the tibia. However, the ease of luxation on palpation of the patella is much more important diagnostically than is the location of the patella on a single craniocaudal radiograph. The affected patella is often in a normal position for a given radiograph if it is not purposely luxated by the examiner before the radiograph is taken.

Surgery is usually indicated for young animals with congenital patella luxation. Most young animals respond well to imbrication of the fibrous joint capsule and overlying fascia on the side opposite the direction of patella luxation. However, the veterinarian must fully evaluate the limb before surgery and assess the joint at surgery. Some severe cases may require trochleoplasty or tibial crest osteotomy and relocation. The reader should refer to small animal surgery texts for detailed descriptions of the more complex stifle surgeries.7

Affected animals should be thoroughly examined for other congenital abnormalities. Specifically, severely affected newborns may not be able to stand and suckle. Therefore failure of passive transfer and associated illness may become more significant to the health of these animals than even the primary patella luxation. Small ruminants may compensate for mild cases of patella luxation (especially if the condition is unilateral) and go undiagnosed until they are seen by veterinarians as adults with lameness caused by luxation or degenerative joint disease caused by intermittent luxation. Adult animals also may exhibit acute lameness as a result of traumatic patella luxation. Surgical treatment of these adults tends to be more involved in that orthopedic implants such as screws and wires may be required to secure the patella. The prognosis for a return to soundness is not good compared with the prognosis for treated neonates with congenital luxations.8,9

Radiologic Signs

1.

The patella may be in normal position on all views.

If displaced, the patella lies to the medial or lateral side of the femur on the craniocaudal view. The displaced patella may be difficult to demonstrate in young animals before it has become fully mineralized.

On the mediolateral view, the patella is absent from the trochlear groove and lies superimposed on the femoral condyles.

A flexed proximocranial-distocranial (“skyline,” or tangential) view of the distal femoral trochlea will show the displaced patella and possibly a shallow trochlear groove.

Associated bone abnormalities are frequently evident, including a shallow trochlear groove, rotation of the proximal tibia, curvature and rotation of the proximal tibia, and abnormal angulation of the femorotibial articulation. Some or all of these abnormalities may be present.

Secondary changes associated with degenerative joint disease may be present.

Luxation may also be the result of trauma, in which case it may be either medial or lateral.

Care should be taken not to rely entirely on the radiographic diagnosis. A luxating patella may be in its normal position when a radiograph is made because positioning the animal for radiography may bring about a temporary reduction of the luxation (Figure 4-11, L and M).