classification of patellofemoral disorders

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  • Arthroscopy: The Journal of Arthroscopic and Related Surgery 4(4):235-240 Published by Raven Press, Ltd. 1988 Arthroscopy Association of North America

    Classification of Patellofemoral Disorders

    Alan C. Merchant, M.D.

    Summary: Patellofemoral disorders represent a large portion of the average orthopedist's practice. Despite the improvements in patellofemoral radio- graphs and arthroscopic diagnostic techniques, these disorders are too fre- quently misunderstood and frustrating to treat. This report proposes a clinical classification for patellofemoral disorders that, it is hoped, will aid our under- standing and improve our results of treatment. A major feature of this classi- fication is the recognition that a developmental and familial abnormality, pa- tellofemoral dysplasia, is the etiology for most patellofemorat disorders. Equally important is the assignment of chondromalacia patellae to a secondary position for the most part. Other causes of anterior knee pain and disability are included to complete the classification. Key Words: Classification-- Patellofemoral disorders--Patella--Patellofemoral dysplasia--Chondro- malacia patellae--Recurrent dislocation of the patella.

    Editor's comments: This article by Dr. Alan C. Mer- chant was written at the request of the editorial office of Arthroscopy, with the hope that it will help clarify the various diagnoses and the treatment of disorders of the patellofemoral joint. After publica- tion of this article, we hope that chondromalacia of the patella will no longer be considered synony- mous with the vague anterior knee pains that are so commonly seen in the young patient. By virtue of his clinical experience and the research that he has done, no one is better qualified to write on this sub- ject than Dr. Merchant.

    Our comprehension and treatment of patellofem- oral disorders suffer from the lack of a generally accepted classification and definition of terms. Un- til such a classification is in common use, it will remain difficult to understand, diagnose, and treat these conditions. Worse yet, without clearly de- fined diagnostic categories, a symptom such as "an-

    From the Department of Orthopedic Surgery, El Camino Hos- pital, Mountain View, and Division of Orthopedic Surgery, De- partment of Surgery, Stanford University School of Medicine, Stanford, California.

    Address correspondence and reprint requests to Dr. A. C. Merchant at 2500 Hospital Dr., Bldg. 7, Mountain View, CA 94040, U.S.A.

    terior knee pain" (1,2) becomes accepted as a diag- nosis, adding to the confusion.

    A clinical classification should perform at least two functions. First, it should help the clinician se- lect the proper treatment plan and second, it should provide clearly defined diagnostic categories so that results can be compared retrospectively and pro- spectively. If the categories are too vague or broad, such as "internal derangement," the tendency is to stop looking for the etiology once the label has been applied. This leads to the treatment of symptoms using rote techniques rather than individualizing the treatment based on the causative factors present and each patient's needs.


    A proposed clinical classification of patellofem- oral disorders is presented in Table 1. This classifi- cation is designed for clinical use and based on eti- ology. Usually, the clinician can establish a diagno- sis or differential diagnosis using the history, physical examination, and routine radiographs. Furthermore, by focusing upon etiologies rather than symptoms such as anterior knee pain or sec- ondary changes such as chondromalacia, a rational treatment program naturally follows.


  • 236 A. C. MERCHANT

    TABLE 1. Classification of patellofemoral disorders

    I. Trauma (conditions caused by trauma in the otherwise normal knee) A. Acute trauma

    1. Contusion 2. Fracture

    a. Patella b. Femoral trochlea c. Proximal tibial epiphysis (tubercle)

    3. Dislocation (rare in the normal knee) 4. Rupture

    a. Quadriceps tendon b. Patellar tendon

    B. Repetitive trauma (overuse syndromes) 1. Patellar tendinitis ("jumper's knee") 2. Quadriceps tendinitis 3. Peripatellar tendinitis (e.g., anterior knee pain of

    the adolescent due to hamstring contracture) 4. Prepatellar bursitis ("housemaid's knee") 5. Apophysitis

    a. Osgood-Schlatter disease b. Sinding-Larsen-Johanssen disease

    C. Late effects of trauma 1. Posttraumatic chondromalacia patellae 2. Posttraumatic patellofemoral arthritis 3. Anterior fat pad syndrome (posttraumatic

    fibrosis) 4. Reflex sympathetic dystrophy of the

    patella 5. Patellar osseous dystrophy (11) 6. Acquired patella infera 7. Acquired quadriceps fibrosis

    II. Patellofemoral dysplasia A. Lateral patellar compression syndrome

    1. Secondary chondromalacia patellae 2. Secondary patellofemoral arthritis

    B. Chronic subluxation of the patella 1. Secondary chondromalacia patellae 2. Secondary patellofemoral arthritis

    C. Recurrent dislocation of the patella 1. Associated fractures

    a. Osteochondral (intraarticular) b. Avulsion (extraarticular)

    2. Secondary chondromalacia patellae 3. Secondary patellofemoral arthritis

    D. Chronic dislocation of the patella 1. Congenital 2. Acquired

    III. Idiopathic chondromalacia patellae IV. Osteochondritis dissecans

    A. Patella B. Femoral trochlea

    V. Synovial plicae (anatomic variant made symptomatic by acute or repetitive trauma) A. Medial patellar ("shelf") B. Suprapatellar C. Lateral patellar

    I would propose that the term "chondro- malacia" never be used alone as a diagnosis, but always be qualified with the words "secondary" or "idiopathic." For example, a diagnosis of "chronic patellar subluxation with secondary chondro-

    malacia" is much more accurate than "chondroma- lacia patellae" alone. It also defines the etiology. It was Aleman (3) in 1928 who first used the term "chondromalacia" in the diagnosis of "chondroma- lacia post-traumatica patellae" to describe an artic- ular lesion of the patella caused by prior trauma and found at surgery. Unfortunately, as the years went by, "chondromalacia patellae" was used alone as a diagnosis without reference to etiology and gradu- ally became equated with anterior knee pain. We are only now emerging from this confusion.

    The first section of this classification, listing con- ditions caused by trauma whether acute, repetitive, or delayed, is self-explanatory for the most part and requires little comment. The second section dealing with enigmatic patellar pain and instability will be discussed below. Section three allows inclusion of those cases of proven chondromalacia patellae for which no cause can be found. The last two sections complete the classification with no attempt to in- clude rare entities such as tumors, infections, or metabolic disorders.

    Following the use of accurate axial view patello- femoral radiographs, it became apparent that a "normal" knee almost never suffers a dislocation of the patella. Rarely, a direct glancing blow to the medial edge of the patella causes a dislocation in an otherwise healthy knee. Similarly, the increased valgus and external rotation deformity of the tibia associated with an acute ligament rupture of the knee can also rupture the vastus medialis obliquus (VMO) and cause a patellar dislocation. Almost al- ways the patient who suffers from patellar instabil- ity has an abnormal patellofemoral articulation or extensor mechanism to begin with. The only way to discover these abnormalities is by using a careful history and physical examination supplemented by accurate radiographs taken in the proper positions. A distorted radiograph will yield distorted informa- tion.


    The pattern of preexisting abnormalities, such as a shallow intercondylar sulcus, deficient VMO, pa- tella alta, chronic patellar subluxation, etc., sug- gests underlying genetic, developmental, and famil- ial abnormalities. The expression of these abnor- malities can vary from mild to severe, but it is helpful to apply the overall and unifying term: "pa- tellofemoral dysplasia." This can be thought of as

    Arthroscopy, Vol. 4, No. 4, 1988


    analogous to congenital dysplasia of the hip. Em- bryologically, the femoral trochlea develops early in utero (8 weeks) complete with its adult predom- inance of the lateral condyle even before articula- tion with the patella occurs. Some have postulated that the trochlea fails to develop owing to a genetic defect. Others (4) feel that dysplasia of the quadri- ceps mechanism is primary, allowing patellar sub- luxation and leading to pressure inhibition of the lateral condyle. From a clinical viewpoint, we are not able to decide this question any more than we can answer, "Which came first, the chicken or the egg?" We should consider this group of patellofem- oral disorders as a developmental dysplasia charac- terized by a continuum of anatomic deficiencies. This will tend to focus our attention on the search for the sometimes subtle abnormalities to explain each patient's symptoms and help individualize our treatment to correct these deficiencies.

    Having combined these conditions together un- der the large category of patellofemoral dysplasia, it is also incumbent upon us to subdivide this classi- fication. This will allow those patients with symp- toms, signs, and severity in common to be assessed as a group. Different treatment protocols can be developed and compared. To do this, we must eval- uate each of the factors associated with patellofem- oral disorders known to influence patellar instabil- ity and pain. The normal lateral vector imparted to the patella by the no


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