339

Dorsal Defect of thePatella: Incidence and Distribution

John F. Johnson1

B. G. Brogdon1

To determine the incidence and distribution of dorsal defect of the patella, theradiographs of 1 ,349 knees in 1 ,192 consecutive patients were evaluated. The defectwas present in about 1 % of the population. It may be found at any age from thepreadolescent to the mature adult. Females were more commonly affected than males.The test population group was validated by concurrent tabulation and comparison withother series of two other benign lesions occurring at the knee: fabella and multipartitepatella.

A recent forensic case in which a dorsal defect of the patella was the key to

identification of carbonized skeletal remains prompted our interest in the fre-

quency and distribution of that lesion. Illustrations of dorsal defect of the patella,

with very brief accompanying statements, appeared first in Caffey [1 ] and

subsequently in Keats [2]. Three other original papers on the subject have been

found [3-5], but none are revealing as to the incidence of the lesion, which

ordinarily is encountered as an incidental radiographic finding. In those cases

previously reported or illustrated, 20 of 26 were male. The youngest patient was

1 0 years of age, the oldest 31.

The dorsal defect has been described as a characteristic, well marginated lyticlesion in the dorsal aspect of the patella, usually lying in the supemolatemal angle

of the bone, although rarely it may tend toward a more central location [3-5] (fig.

1 ).

Materials and Methods

All recoverable radiographic examinations of the knee done at this institution during1 981 were consecutively reviewed. There were 1 349 knees in 1 ,1 92 patients. Each

examination was scrutinized carefully in two or more projections by one of the authors. Allquestionable or equivocal cases were reviewed by both.

As a check on the statistical validity of the patient material available, all instances of twoor more ossification centers in the patella and all findings of a fabella being present wererecorded.

Results

Received February 1 2, 1982.; accepted afterrevision April 1 9, 1982.

1 Department of Radiology, University of South

Alabama Medical Center, 2451 Fillingim St., Mo-bile, AL 3661 7. Address reprint requests to B. G.Brogdon.

AJR 1 39:339-340, August 19820361 -8o3x/82/1 392-0339 $00.00© American Roentgen Ray Society

Thirteen dorsal defects of the patella were found in 1 2 individuals. They were

1 7-56 years old. Ten were females; only four were black. When both knees were

available for evaluation, the lesion was bilateral in only one of three affected

individuals.

Two on more ossification centers for the patella were found in 1 .4% of knees

studied. When both patellae were available for evaluation, bilatenality of this

finding was present in three of four individuals. Theme was no significant differ-

ence in the distribution of multiple ossification centers of the patella by gender on

A B

340 JOHNSON AND BROGDON AJR:139, August 1982

Fig. 1 -Frontal (A) and lateral (B) projections of patella show characteristic dorsal defect (arrows) of patella in typical location. This was an incidentalfinding in a 21-year-old white man with mid-shaft fracture of ipsilateral femur.

mace.

A fabella was found in 1 28 (1 0%) of 1 304 knees. The

youngest patient with a fabella was 1 4 years; the oldest 83.

When ascertainable, bilaterality was found in 24 (83%) of

29 individuals.

Discussion

Our findings with respect to multipartite patellae and

fabellae are consistent with other reports and seem to

confirm the validity of our series [6-8].

Our discovery of 1 3 cases of dorsal defect of the patella

in 1 349 knees demonstrates an incidence of that lesion in

1 % of the population. All of the cases tabulated fit the

radiographic description of dorsal defect of the patella, and

no other differential diagnosis was entertained seriously in

any of them. Three other possible or probable cases were

not counted because the defect conceivably could be attnib-

uted to degenerative arthritis in two patients and to a brown

tumor of hypempamathymoidism in the other. Contrary to other

published series, we found a predilection of the lesion for

females (level of significance = 0.05).

Our experience that three-fourths of the cases of dorsal

defect of the patella were discovered fortuitously is con-

sistent with others. One patient with long-standing com-

plaints refenmable to the right knee underwent a normal

arthmoscopy, and the asymptomatic knee was found to con-

tam an identical lesion. The other two symptomatic patients

had vague arthnalgias not limited to the affected knee. In

only one reported case [5] was the defect thought to be

directly related to the patient’s symptoms.

The etiology of dorsal defect of the patella is not known.That it is simply a developmental alteration of the epiphysis

with delayed or failed ossification is the most attractive

possibility. The occurrence of the lesion in that area of the

patella most prone to irregularity on multiplicity of ossifica-

tion centers seems to further support this possibility. Some

dorsal defects have shown evidence of healing or filling-in

over a period of observation [2, 4]. In other individuals, asour series shows, dorsal defect of the patella may persistlate into adult life. This is not surprising and is somewhat

analogous to the multipartite patella, wherein many individ-

uals start with multiple ossification centers in the patella, but

only a few will demonstrate a multipartite patella as an adult.

Histologic material, in the few cases biopsied [4, 5], hasnot been helpful in elucidating the etiology of dorsal defect

of the patella, showing primarily nonspecific fibrous tissue

with no evidence of inflammation or specific reaction.

Thus, in virtually all cases, the dorsal defect of the patella

can be confidently identified and diagnosed in about one of

every 1 00 individuals. Ordinarily, no further investigation,

procedure, on treatment will be indicated or necessary.

ACKNOWLEDGMENT

We thank Donald E. Herbert for statistical analysis of some of the

data.

REFERENCES

1 . Caffey J. Pediatric x-ray diagnosis, 6th ed. Chicago: Year Book

Medical, 1972:943

2. Keats TE. An atlas of normal roentgen variants, 1 st ed. Chi-

cago: Year Book Medical, 1973:200

3. Goergen TG, Resnick D, Greenway G, Saltzstein SL. Dorsal

defect ofthe patella (DDP): a characteristic radiographic lesion.

Radiology 1 979; 1 30 : 333-3364. Haswell DM, Berne AS, Graham CB. The dorsal defect of the

patehla. Pediatr Radio! 1 976:4 : 238-2425. Hunter LY, Hensinger RN. Dorsal defect of the patella with

cartilagenous involvement: a case report. Clin Orthop

1975;1 10:131 -132

6. Ficat AP, Hungerford DS. Disorders ofthe pate!lo-femora! joint.

Baltimore: Williams & Wilkins, 1977:56-57

7. Smillie IS. Diseases of the knee joint, 2d ed. New York:

Churchill-Livingston, 1980:49-50

8. Friedman AC, Naidich TP. The fabella sign: fabella displace-

ment in synovial effusion and pophiteal fossa masses. Radiology

1978;127:113-121