Transcript
Page 1: Classification of patellofemoral disorders

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.

PROPOSED CLINICAL CLASSIFICATION

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.

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TABLE 1. Classification o f 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.

PATELLOFEMORAL DYSPLASIA

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

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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 normal quadriceps angle (Q angle) is resisted by (a) the depth of the femoral trochlea with its larger lateral condyle and (b) the VMO whose fibers insert more distally and horizontally on the patella than those of the vastus lateralis. A deficiency of either the intercondylar sulcus or the VMO or both predisposes to patellar subluxation and dislocation. Any increase in the Q angle itself from any cause (internal femoral torsion, external tibial torsion, genu valgum, etc.) increases the lat- eral vector on the patella. There is also a dynamic increase in the Q angle when the foot is planted and the femur internally rotates during the common ma- neuver of cutting and pushing off, predisposing to dislocation. The lateral tethering of a tight lateral retinaculum can also increase the lateral force on the patella.

A high-riding patella (patella alta) will also in- crease patellar instability since it articulates in the more shallow superior portion of the sulcus for any given degree of knee flexion compared with the nor- mal. That is, the knee must be flexed more to bring

a high-riding patella safely within the deeper portion of the trochlea.

Lateral patellar compression syndrome The mildest form of patellofemoral dysplasia is

the lateral patellar compression syndrome (LPCS) described by Ficat et al. (5) in 1975. It is character- ized by patellar pain while the patella remains stable within the sulcus without evidence of subluxation. The sine qua non is functional lateralization of the patella by increased lateral forces, decreased me- dial forces, or a combination of both. Thus, by def- inition, LPCS should have the following elements. A history is obtained of anterior knee pain usually aggravated by flexed knee activities without epi- sodes of true dislocation or subluxation. Examina- tion should reveal an increased Q angle either static, dynamic, or both. Accurate axial view radio- graphs show no patellar subluxation when taken with the knees flexed 30 ° . The longstanding effect of increased lateral pressure frequently produces scle- rosis of the subchondral bone under the lateral pa- tellar facet (Fig. 1).

While not a true dysplasia of the patellofemoral joint, the rotational malalignment of the extensor mechanism produced by combined internal femoral torsion and compensatory external tibial torsion creates an increased lateral patellar compression. Theretbre, it is placed within this category.

Chronic Subluxation of the Patella The next more severe manifestation of patello-

femoral dysplasia is chronic subluxation of the pa- tella (CSP). In this condition, the patella remains chronically displaced from its normal position dur-

FIG. 1. Lateral patellar compression syndrome. There is no sub- luxation of the patella on an axial radiograph taken at 30 ° of knee flexion. Subchondral sclerosis of the lateral patellar facet is fre- quently present.

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ing at least the first part of its trochlear excursion. As in LPCS, the presenting symptom is usually an- terior knee pain aggravated by flexed knee activi- ties. Again, on examination, an increased Q angle is evident. But the difference lies in the axial radio- graphs: Patellar subluxation is present on films taken with the knees flexed 30 ° or more (Fig. 2).

A word of explanation is needed here to justify the apparent arbitrary division between LPCS and CSP. Since patellofemoral dysplasia is really a con- tinuum of abnormalities, there can be no true divi- sion. However, a separation of such a large class of patients, even arbitrarily, into groups with similar findings and severities is helpful when planning treatment and assessing results. The most sensitive measure of patellar subluxation that is readily avail- able to the clinician is the axial radiograph. Ficat et al. (6), Merchant et al. (7), and Labelle et al. (8) have all described accurate techniques to obtain ax- ial view radiographs. We advocate an initial screen- ing view taken at 45 ° knee flexion, since this degree of flexion makes an easy exposure for the techni- cian and most subluxations will be seen. If no sub- luxation is evident on the 45 ° flexion view, a 30 ° view is taken and this will occasionally demonstrate a patellar subluxation not evident at 45 ° (Fig. 2). Obtaining an axial view with the knees flexed <30 ° is technically very demanding and so difficult as to be unrewarding in a clinical practice. Certainly, the patellofemoral relationship from 0 to 30 ° of knee flexion can be assessed with computerized tomog- raphy scans or magnetic resonance imaging but the radiation exposure, time, and expense involved ob- viate these techniques as a basis for clinical classi- fication.

Once the axial radiograph is taken, what is the definition of subluxation? Most of the time, the pa- tellar subluxation is self-evident, but when it is al- most normal, measurement is required. The most sensitive measure of patellofemoral congruence is

the congruence angle. (Fig. 3). In our original study (7), we erroneously assumed that anyone who gave no history of knee pain or knee problems would be "normal ." Thus, our "normals" yielded a mean congruence angle of - 6 ° and an SD of 11 °. When Aglietti et al. (9) repeated our work, they examined all their normal controls and excluded those with physical abnormalities as well as a history of knee problems. It is interesting that their mean congru- ence angle was approximately the same at - 8 °, but their SD was nearly 50% smaller at 6 °. For these reasons, I consider their values more accurate. A subluxation with a congruence angle of greater than + 4 to + 6 ° is abnormal at the 95th percentile. This difference between these two studies, one using only the history and the other using both the history and the physical examination to determine "nor- mal" controls, demonstrates just how prevalent asymptomatic patellofemorai dysplasia is in the general population.

Recurrent Dislocation of the Patella The most severe form of patellofemoral dysplasia

commonly seen in clinical practice is recurrent dis- location of the patella (RDP) in which the patient suffers episodic patellar instability. Clinically, one should be very careful to distinguish between the sudden feeling of weakness or release of the quad- riceps that can be induced by sudden patellar pain and the true collapse of the knee as the patella dis- locates. The patient will describe both as "going out" or "giving way." If the axial radiographs do not show patellar subluxation or a shallow sulcus angle or both, then the diagnosis of RDP should be seriously questioned (Fig. 4).

Concerning the diagnosis of recurrent sublux- ation of the patella, I would agree with Brattstrom (10) that since the difference between a momentary subluxation and a dislocation is one of degree only and cannot be quantified, only the term "recurrent

FIG. 2. Chronic subluxat ion of the patella. A: Subluxation of the patella is present on an axial radiograph taken at 30 ° of knee flexion. B: At 45 ° of knee flexion, the same patella is reduced.

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- o +

I

FIG. 3. The sulcus angle is angle MSL (mean = 137 °, SD = 6 °. The zero reference line, SO, bisects the sulcus angle. The con- gruence angle is angle ASO (mean = - 6 to - 8 °, SD = 6°; see text).

dislocation" should be used as a diagnostic cate- gory. Since these three diagnoses, LPCS, CSP, and RDP, all have common features and are more or less continuous, any given patient can change from one diagnosis to the next with the passage of time, change of activity level, injury, or treatment. For instance, a patient under successful treatment for LPCS could change sports, suffer an injury, and experience an initial dislocation of the patella. If the injury tears the medial retinaculum, chronic sublux- ation could result, which was not present before. Similarly, a patient who undergoes a successful lat- eral release for CSP could be left with mild symp- toms and a diagnosis of LPCS despite the correc- tion of the patellar subluxation radiographically.

Chronic Dislocation of the Patella The most severe form of patellofemoral dysplasia

is chronic dislocation of the patella in which the patella never returns to the trochlea throughout the range of motion, thus remaining permanently dislo- cated. Fortunately, this is a rare condition. There

FIG. 4. Recurrent dislocation of the patella. Axial radiograph of a 17-year-old female gymnast whose right patella would dislocate only during a faulty dismount. The patella is congruent, but the sulcus is abnormally shallow.

FIG. 5. Chronic dislocation of the patella, acquired. Axial ra- diograph of a 57-year-old woman who had a patellar dislocation 7 months before.

are congenital and acquired forms. The congenital form is felt to be an external malrotation of the en- tire quadriceps muscle mass and the extensor mech- anism on the femur. The acquired form can be seen secondary to fibrosis of the vastus tateralis when multiple intramuscular injections are given to an in- fant. On rare occasions, a chronic dislocation is ac- quired following a patellar dislocation (Fig. 5).

CONCLUSION

If the concept of patellofemoral dysplasia is ac- cepted, then purely traumatic conditions of the pa- tellofemoral joint are those that occur in an other- wise normal knee. Obviously, trauma can be super- imposed on a dysplastic knee as well.

In this classification of patellofemoral disorders, the term "chondromalacia patellae" is omitted as a diagnosis by itself. It resumes its proper meaning as a descriptive term for a lesion of articular cartilage secondary to other causes. For that steadily shrink- ing group of patients in whom no causative factor can be determined, the diagnosis of idiopathic chon- dromalacia patellae is used.

Other conditions affecting the patellofemoral joint require separate categories. These are osteo- chondritis disecans and synovial plicae.

It is hoped that this classification will help clini- cians avoid confusion and encourage them to seek the cause of each patient's complaints to develop and individualize a rational treatment protocol rather than focus mainly on the treatment of symp- toms.

REFERENCES

1. Heatley FW, Allen PR, Patrick JH. Tibial tubercle advance- ment for anterior knee pain; a temporary or permanent so- lution. Clin Orthop 1986;208:215-24.

2. Miller B J, LaRochelle PJ. The treatment of pateUofemoral

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pain by combined rotation and elevation of the tibial tuber- cle. J Bone Joint Surg [Am] 1986;68:41%23.

3. Aleman O. Chondromalacia post-traumatica patellae. Acta Chir Scand 1928;63:194.

4. Fox TA. Dysplasia of the quadriceps mechanism. Surg Clin North Am 1975;55:199.

5. Ficat P, Ficat C, Bailleux A. Syndrome d'hyperpression ex- terne de la rotule (S.H.P.E.). Rev Chit Orthop 1975;61:3% 59.

6. Ficat P, Phillipe J, Bizou H. Le defile femeropatellaire. Rev Med Toulouse 1970;6:241-4.

7. Merchant AC, Mercer RL, Jacobsen RH, Cool CR. Roent- genographic analysis of patellofemoral congruence. J Bone Joint Surg [Am] 1974;56:1391-6.

8. Labelle M, Peides JP, Levesque HP, Fauteux P, Laurin CA. Evaluation de la position rotulienne en incidence radio- graphique tangentielle. Union Med Can 1976;105:870.

9. Aglietti P, Insall JH, Cerulli G. Patellar pain and incongru- ence. I: measurements of incongruence. Clin Orthop 1983;122:217-24.

10. Brattstrom H. Shape of the intercondylar groove normally and in recurrent dislocation of the patella; a clinical and x-ray anatomical investigation. Acta Orthop Scand [Suppl] 1964;68: t-148.

11. Dye SF, Boll DA. Radionuclide imaging of the patellofem- oral joint in young adults with anterior knee pain. Orthop Clin North Am 1986;17:249-62.

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