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
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females (level of significance = 0.05).
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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 patientâs 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