CHAPTER 9 Arthroscopic Evaluation and Diagnosis of the Patellofemoral Joint
Historically, arthroscopy was used as a diagnostic modality. With the development of less invasive imaging technology (magnetic resonance imaging [MRI] and computed tomography [CT]), it is now rarely used as a purely diagnostic tool. Frequently, the exact cause of patellofemoral pain or maltracking is unknown and arthroscopy can help in the diagnosis and treatment of these problems. Causes of patellofemoral problems vary and include instability, maltracking, degenerative arthritis, and traumatic injury. With the advent of arthroscopy, orthopedic surgeons have added a significant tool to their diagnostic and therapeutic armamentarium.
History and physical examination continue to be the mainstay in the diagnosis of anterior knee pain. X-rays, CT, and MRI are all helpful imaging modalities to confirm possible diagnoses. Arthroscopy can also help in the confirmation and localization of patellofemoral lesions and in ruling out other pathologic lesions that may be responsible for the symptoms. However, it must also be mentioned that there are many pathologic lesions seen on arthroscopy that are asymptomatic. Primary complaints by patients can be pain and/or instability caused by patellar dislocations, maltracking, overuse syndromes, tendinitis, localized trauma, saphenous neuritis, and apophysitis.
Patellofemoral pain usually presents as diffuse anterior knee pain. The pain generally worsens with activity and improves with rest. Most commonly, the onset is gradual and associated with physical activity. Chondral injury is usually worse during the activity and improves on stopping, whereas tendinitis improves with activity, but worsens in the period following activity. Also, tendinitis results in tenderness localized to the area of tendinitis (patellar tendon, quadriceps, iliotibial band).
Instability can result from pain-induced quadriceps inhibition secondary to meniscal pathology or ligamentous instability. This should be differentiated from patellofemoral instability. Patellar subluxation and dislocation can be differentiated from each other by weakness and effusion being prevalent with patellar dislocation. Only truly unstable subluxation results in abnormal motion of the patellofemoral joint. Crepitus and swelling are other common complaints indicating chondral injury.
Differentiating acute from chronic problems, the quality of the patient’s pain, and what makes it better and worse all help the clinician make the diagnosis. Table 9-1 presents the major subjective differences when taking a history of a patient’s problem with patellofemoral complaints.
The physical examination should be performed with the goal of systematically evaluating the entire lower extremity. Patellofemoral problems can stem from malalignment secondary to femoral anteversion, genu recurvatum, genu varum, genu valgum, tibial torsion, pes planus, or ligamentous laxity. The extensor mechanism should be evaluated, as well as patellar position, mobility, and tracking.
Patellofemoral function and pathology are dependent on bony anatomy, lower limb alignment, and soft tissue restraints, including muscle function and ligamentous stability. Start with a general assessment of the patient’s condition, ligamentous laxity, and overall posture. Perform a vascular examination of the lower extremities, including posterior tibial and dorsalis pedis pulses. A neurologic evaluation of sensation, motor strength, and reflexes should also be performed. Vascular and, more important, neurologic disorders can lead to patellofemoral problems that are not as easily treated and need to be identified to be managed appropriately.
Next, the examiner should evaluate the joints above and below the knee (hip along with the foot and ankle). It is easy to focus on the knee and patellofemoral joint. Examine all aspects of the knee prior to focusing on the suspected patellofemoral problem (Table 9-2).
|Observation||Alignment||Varus, valgus, patella (alta, baja, or squinting), decreased foot progression angle, pes planus (increased forefoot pronation)|
|Palpation||Knee joint||Effusion, tenderness, crepitus, pain with patellar compression on ROM|
|Range of motion (ROM)||Passive and active||Amount of ROM, limited end points, extensor lag|
Lateral → translation of > 50% of patella width is abnormal; if no hard end point, MPFL (53% of lateral stability) is out
ASIS, anterosuperior iliac spine.
The anatomical landmarks of the patellofemoral joint are the seven facets of the posterior patella (Fig. 9-1). The medial and lateral facets are each divided into roughly equal thirds and the seventh facet is the odd facet located on the extreme medial border of the patella. On the trochlear side, the patella articulates with the femoral joint surface on the medial and lateral sides of the trochlear groove. The contact area pressures vary in location and load depending on the level of flexion. The patellofemoral contact, load, and tracking are also affected by anatomical variants, as described by Wiberg and Baumgartl (Fig. 9-2). Also, anatomic abnormalities such as patella baja, patella alta, trochlear dysplasia, rotational alignment of the femur and tibia, foot alignment, tight lateral retinaculum, and vastus medialis strength also play a role in the tracking and articulating pressures and contact locations (Fig. 9-3).
FIGURE 9-1 Seven facets described on the articular side of the patella. Illustrated are the superior medial facet (SMF), middle medial facet (MMF), inferior medial facet (IMF), superior lateral facet (SLF), middle lateral facet (MLF), inferior lateral facet (ILF), and odd facet.
FIGURE 9-2 Anatomic variants of the patella as described by Wiberg and Baumgartl.23 I. Equal medial and lateral facets which are both slightly concave (normal anatomy), II. Smaller medial than lateral facet, medial facet is flat or slightly convex, III. Very small medial facet which is convex, IV. Without a medial ridge or medial facet.
FIGURE 9-3 Anatomy of the medial aspect of the knee. The MPFL provides 53% of the restraint to lateral displacement of the patella. The patellomeniscal ligament and medial retinacular fibers are responsible for 22% of patellar restraint.
Alignment can also be assessed with the Q angle. As the knee angle changes the Q angle changes. It is largest with the knee in full extension. This has been proposed to be the reason for patellar subluxation/dislocation with the deforming force being greatest in the most unstable position. The foot-up model of the dynamic Q angle is based on the premise that increased foot pronation results in tibial internal rotation producing an in-turned knee posture and “functional” valgus.
Anatomic variations responsible for patellofemoral problems vary, depending on the pathology. Excessive femoral anteversion, tibial torsion, patella alta, and shallow trochlea are all causes of patellar instability, and pain caused by instability (Fig. 9-4).
FIGURE 9-4 Trochlear dysplasia as described by Dejour. Type A, Shallow trochlea. Look for the crossing sign on the lateral x-ray. Type B, Flat trochlea. Look for trochlear spur on lateral x-ray. Type C, Lateral convexity and medial hypoplasia. Look for double contour on lateral. Type D, Cliff type; supratrochlear spur and double contour on lateral x-ray.
Proper diagnostic imaging is very important in the diagnosis of the root cause of the problem. Patellofemoral pain versus instability can have similar causes. Limb alignment may be assessed on clinical examination. The first images that should be obtained because of availability and ease of use are x-rays. Obtaining standing anteroposterior (AP) lateral views, skyline views of the patellofemoral joint, and standing notch views help in assessing the bony anatomy of the knee. Standing long leg views are important to obtain to assess alignment. Long leg views can assess varus and valgus malalignment, and skyline views can assess patellofemoral cartilage thickness, gross patellar tracking, and whether there is increased patellar tilt. However, routine radiographs have been shown to identify fewer than 50% of osteochondral loose bodies.
When the patella does not engage in the trochlea by 15 to 20 degrees of knee flexion, patella alta may be present. Patella alta may be radiographically assessed in many ways. Laurin and Merchant axial radiographs are obtained with the knee flexed 20 and 40 degrees, respectively (Fig. 9-5). These assess patellar tilt, but one tangential radiograph obtained at 30 degrees of flexion is sufficient in most cases. The x-ray beam is projected caudad at an angle of 30 degrees from the plane of the femur. A line is drawn along the lateral facet of the patella, and a second line is drawn between the condyles of the trochlea anteriorly. Normally, the angle between these two lines will be open laterally. However, if the lines are parallel or the angle opens medially, the patella is probably tilted.
FIGURE 9-5 A, Diagram of the Merchant skyline radiographic view. The sulcus angle ABC is bisected by BD and line BE passes from the deepest point of the sulcus through the most posterior point on the ridge of the patella. The positive congruence angle DBE indicates lateral translation. B, The Laurin angle is negative, indicating patellar lateral tilt.
Teitge and colleagues1 have described a radiographic technique that can be helpful for diagnosing patellar instability. They obtained bilateral axial radiographs of the patellofemoral joints in anatomic position, with constant medial and lateral force applied to the patellae with an instrumented device, and then axial radiography was repeated. It was found that a 4-mm increase in medial or lateral patellar excursion compared with the patellar excursion on the asymptomatic knee correlated with patellar instability. Stress radiographs are helpful in identifying patients with congruity of the articular surfaces whose knees may subluxate or dislocate because of deficient ligamentous structures. Patients who are unable to relax the extensor mechanism because of pain or who have bilateral symptoms are not candidates for stress radiography.
Once x-rays have been evaluated, limb alignment may be further evaluated for rotational malalignment with an overlay alignment CT scan to measure femoral anteversion and tibial torsion. CT has been shown to be more sensitive than axial radiography in delineating patellar malalignment2; it allows axial cuts of the patellofemoral articulation at angles less than 20 degrees of knee flexion, which enhances the detection of subluxation as the patella loses the stabilizing function of the lateral femoral condyle. Another role for CT is in identifying lateralization of the tibial tubercle, as measured by the distance between the tibial tubercle and the trochlear sulcus (Fig. 9-6). An axial CT image demonstrating the femoral trochlear groove is superimposed on an axial image of the tibial tubercle. A line is drawn on this superimposed image between the posterior margins of the femoral condyles. Two lines are drawn perpendicular to this line, one bisecting the femoral trochlear groove and the other bisecting the anterior tibial tuberosity. The distance between these two lines determines the extent of lateralization of the tibial tubercle. Values greater than 9 mm have been shown to identify patients with patellofemoral malalignment, with a specificity of 95% and a sensitivity of 85%.3