Technical Note

The Dimpling Phenomenon: Articular Cartilage Injury Overlying an Occult Osteochondral Lesion at the Time of

Anterior Cruciate Ligament Reconstruction

Michael J. Coen, M.D., David N. M. Caborn, M.D., and Darren L. Johnson, M.D.

Summary: The authors report the arthroscopic finding of articular cartilage "dim- piing" when a probe is placed onto the discrete chondral area involved over the geographic bone bruise incurred during traumatic anterior cruciate ligament disruption. As we develop an understanding of the pathology and sequelae of this osteochondral injury, this finding may be useful to document injury extent and possibly guide treatment including weight-bearing status and rehabilitation. Key Words: ACL injury--Bone bruise--Cartilage integrity--Probing--"Dim- piing."

A nterior cruciate ligament (ACL) disruption is a common injury in today's athletic population. In

viewing the long-term results of these injuries with and without surgical intervention, we have gained in- sight into many aspects of the knee. These include its biomechanics, kinematics, tissue properties, aging, and probably most importantly the knee's response to the trauma to which it has been subjected. An appreciation for the intricacies of the individual components in the global picture of what is entailed in a "heal thy" knee continues to be defined. One of the more recent areas of interest is that of the "bone bruise" seen in acute ACL injuries. The underlying pathology of this lesion is just beginning to be realized and understood. Recent information has been published on its radiographic ~9 and arthroscopic ~°-H evidence. This report discusses a phenomenon we have observed arthroscopically (and termed "dimpling") that may aid in evaluating the

From the Section of Sports Medicine, Division of Orthopaedics, the University of Kentucky, Lexington, Kentucky, U.S.A.

Address correspondence and reprint requests to Michael J. Coen, M.D., University of Kentucky Sports Medicine Center, Kentucky Clinic K436, Lexington, KY 40536-0284, U.S.A.

© 1996 by the Arthroscopy Association of North America 0749-8063/96/1204-143153.00/0

extent and possibly degree of osteochondral involve- ment associated with this injury. The long-term se- quelae and pathodegenerative implications of the "bone bruise" remain to be elucidated.

Magnetic resonance imaging (MRI), radiographic, and arthroscopic studies after ACL injury have shown a plethora of concurrent meniscal and osteochondral lesions. Bone bruises are evident in many sites with the sulcus terminalis of the lateral femoral condyle being the area most commonly involved. Injury mecha- nisms 6 resulting in specific lesion locations have been proposed, especially anterior subluxation with tibial impaction on the femur causing a corresponding tibial bone bruise. Arthroscopic evaluation of the articular surfaces have shown cartilage lesions in 20% to 46%, l~ but correlation with specific MRI lesions has been poor. Spindler et al.l~ reported a significant correlation of arthroscopically visualized articular cartilage dam- age and MRI findings (impacted subchondral bone or abnormal articular cartilage signal) only in the lateral femoral condyle bone bruise.

We have found that while the proportion of visual- ized articular lesions overlying MRI bone bruises is in the range of one third to one half, a higher number will have cartilage compromise that is evident with

502 Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 12, No 4 (August), 1996: pp 502-505

THE DIMPLING PHENOMENON 503

FIG 1. (A) Coronal Tl-weighted image (SE 500/20) shows bone bruise of lateral femoral condyle evidenced by crescent-shaped area of low signal. (B) Sagittal view of the same injury. (C) Coronal turbo T2-weighted image (5300/116 Ef) of same injury more clearly shows bone bruise as area of bright signal.

probing. Specifically, a phenomena is evident (Figs 1 and 2) in which gentle placement of a probe in the area under question reveals softening and a transient indentation or " d i m p l e " of the articular surface that persist after probe removal. This method can be used judiciously to outline the boundaries o f macroscopi- cally compromised cartilage at that time. The involved

area can be compared with the size of the lesion on MRI and documented to possibly aid with long-term prognostic prediction as the significance of these inju- ries becomes evident. We are also currently evaluating arthroscopic instrumentation similar to that used as described by Kiviranta et al. 12 This may allow a more exact quantitative measurement of articular integrity.

504 M. J. COEN E T AL.

FIG 2. (A) Articular surface visually unblemished. (B) "Dimpling" of compromised cartilage determining area of "bone bruise."

D I S C U S S I O N

As the high incidence of bone bruise in acute ACL injuries has become evident, a renewed interest in post- traumatic articular cartilage degeneration has devel- oped. The idea of a single supraphysiological blow 13 to the cartilage surface has been hypothesized as a precursor to early osteoarthritis. This has been aptly modeled in the patellofemoral joint of the dog. 14'15 Donohue et a1.14 noted that "con tused" articular carti- lage may show significant alterations in its histological, biochemical, and ultrastructural characteristics without disruption of the articular surface. They reported the earliest gross changes to be cartilage swelling at 2 weeks after the trauma. Thompson et al. 15 more re- cently reported late changes of superficial clefts similar to fibrillation at 24 weeks after insult. Also, Armstrong and Mow16 and others 17 have discussed whether biome- chanical testing of cartilage properties was a much more sensitive way to evaluate articular integrity than visual or histological appearance, and this question re- mains unresolved. Histological reports, 5 although few and equivocal, are an area of current research including correlation of arthroscopic or MRI findings with micro- scopic evidence of chondrocyte injury or death.

Much work has been done in the area of MRI find- ings associated with bone bruises. Classification and grading systems 1'2'4 have been proposed. The diversity of involvement and the timing of evaluation and inter- vention to insult makes plotting a prognostic course difficult. 3 A three-dimensional representation of time versus integrity with a z axis for extent of initial injury

may be needed. Although reticular lesions tend to re- solve by 3 months, 6 a significant number of geographic lesions have long-term osteochondral sequelae includ- ing osteosclerosis, cartilage thinning, or overt osteo- chondral defect. 4 This posttraumatic cartilage failure may result f rom decreased biomechanical elasticity or reparative ability and/or possibly increased/decreased subchondral bony stiffness/support. 5'18 As we begin to understand more about the long-term sequelae o f these injuries, dimpling may prove to be an early prognostic indicator including guidance in the areas o f weight bearing status and rehabilitaion.

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