The study evaluated 1,349 knee radiographs from 1,192 patients to determine the incidence and distribution of dorsal defects of the patella. The key findings were: 1. Dorsal defects of the patella were present in about 1% of the population studied (13 cases out of 1,349 knees). 2. Females were more commonly affected than males (10 females, 4 males). 3. The lesions ranged in age from 17-56 years old and could persist into adulthood. 4. When both knees were available, the defect was bilateral in only one of three affected individuals.

1. 339
Dorsal Defect of the
Patella: Incidence and Distribution
John F. Johnson1 To determine the incidence and distribution of dorsal defect of the patella, the
B. G. Brogdon1 radiographs of 1 ,349 knees in 1 ,192 consecutive patients were evaluated. The defect
was present in about 1 % of the population. It may be found at any age from the
preadolescent to the mature adult. Females were more commonly affected than males.
The test population group was validated by concurrent tabulation and comparison with
other series of two other benign lesions occurring at the knee: fabella and multipartite
patella.
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 lytic
lesion 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 during
1 981 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. All
questionable or equivocal cases were reviewed by both.
As a check on the statistical validity of the patient material available, all instances of two
or more ossification centers in the patella and all findings of a fabella being present were
recorded.
Results
Thirteen dorsal defects of the patella were found in 1 2 individuals. They were
Received February 1 2, 1982.; accepted after
revision April 1 9, 1982. 1 7-56 years old. Ten were females; only four were black. When both knees were
1 Department of Radiology, University of South available for evaluation, the lesion was bilateral in only one of three affected
Alabama Medical Center, 2451 Fillingim St., Mo- individuals.
bile, AL 3661 7. Address reprint requests to B. G. Two on more ossification centers for the patella were found in 1 .4% of knees
Brogdon.
studied. When both patellae were available for evaluation, bilatenality of this
AJR 1 39:339-340, August 1982
finding was present in three of four individuals. Theme was no significant differ-
0361 -8o3x/82/1 392-0339 $00.00
© American Roentgen Ray Society ence in the distribution of multiple ossification centers of the patella by gender on

2. 340 JOHNSON AND BROGDON AJR:139, August 1982
A B
Fig. 1 -Frontal (A) and lateral (B) projections of patella show characteristic dorsal defect (arrows) of patella in typical location. This was an incidental
finding in a 21-year-old white man with mid-shaft fracture of ipsilateral femur.
mace. over a period of observation [2, 4]. In other individuals, as
A fabella was found in 1 28 (1 0%) of 1 304 knees. The our series shows, dorsal defect of the patella may persist
youngest patient with a fabella was 1 4 years; the oldest 83. late into adult life. This is not surprising and is somewhat
When ascertainable, bilaterality was found in 24 (83%) of analogous to the multipartite patella, wherein many individ-
29 individuals. 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], has
Discussion not been helpful in elucidating the etiology of dorsal defect
Our findings with respect to multipartite patellae and of the patella, showing primarily nonspecific fibrous tissue
fabellae are consistent with other reports and seem to with no evidence of inflammation or specific reaction.
confirm the validity of our series [6-8]. Thus, in virtually all cases, the dorsal defect of the patella
Our discovery of 1 3 cases of dorsal defect of the patella can be confidently identified and diagnosed in about one of
in 1 349 knees demonstrates an incidence of that lesion in every 1 00 individuals. Ordinarily, no further investigation,
1% of the population. All of the cases tabulated fit the procedure, on treatment will be indicated or necessary.
radiographic description of dorsal defect of the patella, and
no other differential diagnosis was entertained seriously in ACKNOWLEDGMENT
any of them. Three other possible or probable cases were We thank Donald E. Herbert for statistical analysis of some of the
not counted because the defect conceivably could be attnib- data.
uted to degenerative arthritis in two patients and to a brown
tumor of hypempamathymoidism in the other. Contrary to other REFERENCES
published series, we found a predilection of the lesion for
1 . Caffey J. Pediatric x-ray diagnosis, 6th ed. Chicago: Year Book
females (level of significance = 0.05).
Medical, 1972:943
Our experience that three-fourths of the cases of dorsal
2. Keats TE. An atlas of normal roentgen variants, 1 st ed. Chi-
defect of the patella were discovered fortuitously is con-
cago: Year Book Medical, 1973:200
sistent with others. One patient with long-standing com- 3. Goergen TG, Resnick D, Greenway G, Saltzstein SL. Dorsal
plaints refenmable to the right knee underwent a normal defect ofthe patella (DDP): a characteristic radiographic lesion.
arthmoscopy, and the asymptomatic knee was found to con- Radiology 1 979; 1 30 : 333-336
tam an identical lesion. The other two symptomatic patients 4. Haswell DM, Berne AS, Graham CB. The dorsal defect of the
had vague arthnalgias not limited to the affected knee. In patehla. Pediatr Radio! 1 976:4 : 238-242
only one reported case [5] was the defect thought to be 5. Hunter LY, Hensinger RN. Dorsal defect of the patella with
directly related to the patient’s symptoms. cartilagenous involvement: a case report. Clin Orthop
1975;1 10:131 -132
The etiology of dorsal defect of the patella is not known.
6. Ficat AP, Hungerford DS. Disorders ofthe pate!lo-femora! joint.
That it is simply a developmental alteration of the epiphysis
Baltimore: Williams & Wilkins, 1977:56-57
with delayed or failed ossification is the most attractive
7. Smillie IS. Diseases of the knee joint, 2d ed. New York:
possibility. The occurrence of the lesion in that area of the Churchill-Livingston, 1980:49-50
patella most prone to irregularity on multiplicity of ossifica- 8. Friedman AC, Naidich TP. The fabella sign: fabella displace-
tion centers seems to further support this possibility. Some ment in synovial effusion and pophiteal fossa masses. Radiology
dorsal defects have shown evidence of healing or filling-in 1978;127:113-121