Biomecanica

The Biomechanics of Femoroacetabular Impingement

Daniel E. Martin, MD, and Scott Tashman, PhD

Femoroacetabular impingement (FAI) is proposed as a possible biomechanical etiology of

early, idiopathic hip osteoarthritis (OA). Two primary mechanisms are proposed: cam

impingement and pincer impingement. In cam impingement, an abnormally shaped or

excessively large femoral head or neck abuts against the anterosuperior acetabulum. In

pincer impingement, overcoverage of the proximal femur by the acetabulum results in

impingement. In severe cases, a contre-coup mechanism results in an anterosuperior

contact point that functions as a fulcrum and produces posteroinferior impingement as the

femoral head is levered out of the acetabulum. However, these proposed mechanisms are

made on the basis of surgical observation rather than in vivo documentation of FAI, and

controversy exists as to whether surgical interventions should be made on the basis of

these theories alone. In this review of FAI biomechanics we discuss the proposed biome-

chanical mechanisms of FAI, the analytical methods currently available to study FAI

biomechanics, and the topics that future biomechanical studies of FAI will need to address.

Ultimately, a better understanding the biomechanics of FAI may help physicians design

interventions that decrease the risk of progression to hip OA.

Oper Tech Orthop 20:248-254 © 2010 Elsevier Inc. All rights reserved.

KEYWORDS

cam impingement, femoroacetabular impingement, hip biomechanics, pincer im-

pingement

F

emoroacetabular impingement (FAI) occurs when the

head or neck, or overcoverage of the proximal femur by

head or neck of the femur abuts against the rim of the

the acetabulum.

acetabulum. The principles of hip impingement are studied

Although these anatomic features can be easily recognized

with regard to total hip arthroplasty, in which components

by the use of readily available imaging techniques, such as

must be designed to minimize wear and dislocation.

Im-

plain radiographs, in vivo characterization of abnormal con-

1 -3

tact between the femur and the acetabulum proves more

difficult. Devising and implementing appropriate surgical in-

pediatric hip disorders, where dysmorphic native anatomy or

terventions, therefore, is also difficult. In this review we aim

surgically altered anatomy provides a readily identifiable

to summarize the proposed biomechanical mechanisms of

source of impingement.

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The recognition of hip impingement in these patient pop-

FAI, the analytical methods currently available to study FAI

ulations has led several authors to examine FAI as a potential

biomechanics, and the topics that future biomechanical stud-

ies of FAI will need to address.

cause of early, idiopathic osteoarthritis (OA) in younger pa-

tients. The work of Ganz et al is particularly instrumental in

defining FAI, as this group has performed surgical disloca-

Proposed Mechanisms of FAI

tion of the hip in several hundred patients with symptomatic

impingement and has meticulously documented their in-

Ganz et al

proposed FAI as a mechanism for the develop-

9

traoperative observations.

These observations provide

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ment of early OA in the absence of dysplasia after performing

the basis for 2 proposed mechanisms of FAI: an abnor-

surgical dislocation of the hip on more than 600 symptomatic

mally shaped (nonspherical) or excessively large femoral

patients. On the basis of the location of labral and articular

cartilage pathology, the authors suggested that FAI occurred

most often in terminal exion and that additional shearing

University of Pittsburgh School of Medicine, Department of Orthopaedic

damage could occur if terminal exion was accompanied by

Surgery, Pittsburgh, PA.

rotation. Furthermore, the authors suggested that the im-

Address reprint requests to Scott Tashman, PhD, University of Pittsburgh

pingement could result from 2 possible morphologic abnor-

Biodynamics Laboratory, 3820 South Water Street, Pittsburgh, PA

malities, the cam lesion and the pincer lesion.

15203. E-mail:

tashman@pitt.edu

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