Biomechanics of the Hip

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Current Concepts With Video Illustration

A Clinically Relevant Review of Hip Biomechanics

Karl F. Bowman, Jr., M.D., Jeremy Fox, B.A., and Jon K. Sekiya, M.D.

Abstract: The hip is a complex anatomic structure composed of osseous, ligamentous, and muscularstructures responsible for transferring the weight of the body from the axial skeleton into the lowerextremities. This must be accomplished while allowing for dynamic loading during activities such asgait and balance. The evaluation of hip pain and periarticular pathology can be challenging becauseof the complex local anatomy and broad differential diagnosis. Recent advancements in the evalu-

ation and surgical treatment of hip pathology have led to a renewed interest in the management of these disorders. An understanding of the basic biomechanical and kinematic function of the hip and theconsequences of associated pathology can greatly assist the orthopaedic surgeon in appropriately diag-nosing and treating these problems. In this review we discuss the basic biomechanical concepts of thenative hip and surrounding structures and the changes experienced as a result of various pathologiesincluding dysplasia, femoroacetabular impingement, labral injury, capsular laxity, hip instability, andarticular cartilage injury. We will also discuss the clinical implications and surgical management of thesepathologies and their role in restoring or preserving the native function of the hip joint.

An understanding of hip joint biomechanics consti-tutes an important background for the diagnosisand treatment of hip disorders. This includes knowl-edge of the kinematics, loading experienced duringstatic and dynamic activities, the transmission of me-chanical stresses between the articulating members of the joint, and the interplay between the various tissuesand structures comprising the hip. This allows theclinician to assimilate the effects of the motions and

deformations resulting from the forces and momentsacting on the joint in the selection and guidance of

appropriate medical interventions. Alterations in theanatomy of the hip through acute injury, chronic de-generation, or surgery can signicantly impact thefunction of the hip during activities. The clinical goalof treatment is to alleviate symptoms of pain andprevent the development or progression of degenera-tive changes in the hip.

The evaluation and management of hip pain in theyoung athletic patient have recently become subjectsof intense interest in the practice of sports medicine.The treatment of these patients is further complicatedby the frequently encountered discrepancy between

the issues that the patient feels are important and theissues considered by the surgeon to be important to thepatient. 1 This is a trend that can partly be attributed tothe increasing recognition of the causes of hip painand the evolution of both surgical and nonsurgicaltechniques. 2 These new techniques have been success-ful for managing the symptoms associated with intra-articular and periarticular pathology such as labraltears, femoroacetabular impingement (FAI), capsularlaxity, and developmental dysplasia. The long-term

From the Department of Orthopaedic Surgery (K.F.B., J.K.S.), Medical School (J.F.), and MedSport (J.K.S.), University of Mich-igan, Ann Arbor, Michigan, U.S.A.

J.K.S. has received support from OrthoDynamix, Jacksonville,FL, exceeding $500 related to this research.

Received November 6, 2009; accepted January 27, 2010. Address correspondence and reprint requests to Jon K. Sekiya,

M.D., MedSport, University of Michigan, 24 Frank Lloyd Wright Dr, PO Box 0391, Ann Arbor, MI 48106-0391, U.S.A. E-mail:[email protected]

2010 by the Arthroscopy Association of North America0749-8063/9654/$36.00doi:10.1016/j.arthro.2010.01.027

Note: To access the videos acco mpanying this report, visit theAugust issue of Arthroscopy at www.arthroscopyjournal.org .

1118 Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 26, No 8 (August), 2010: pp 1118-1129
mailto:[email protected]:[email protected]://www.arthroscopyjournal.org/http://www.arthroscopyjournal.org/http://www.arthroscopyjournal.org/http://www.arthroscopyjournal.org/mailto:[email protected]


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outcomes of these interventions are not fully known,and there is a paucity of studies evaluating the bio-mechanical implications of these disorders. This arti-cle discusses the current biomechanical understandingof both native anatomy and pathology of the hip and

what effects surgical management has on these con-ditions.

FUNCTIONAL ANATOMY ANDKINEMATICS

The hip effectively acts as a multi-axial ball-and-socket joint upon which the upper body is balancedduring stance and gait. Stability of this joint is criticalto allow motion while supporting the forces encoun-tered during daily activity. Nearly all motion betweenthe femoral head and acetabulum is rotational, with nodetectable translation because of the congruency of the articulating surfaces. 3,4 The range of motion re-quired in the hip during everyday tasks, such as risingfrom a chair, lifting weight from a squatting position,walking, stair climbing, mounting a bicycle, and sit-ting cross-legged, can be described with 3 rotationalaxes. 5 This high degree of articular congruency isprovided by the bony architecture of the joint and theacetabular labrum, articular cartilage, joint capsule,and surrounding musculature.

Inherent stability is provided by the osseous anat-omy of the femoroacetabular articulation by the depthof the acetabulum. 4 Although the articular surfaces arevery conforming, a small amount of asymmetry existsbetween the unloaded femoral head and acetabulum,with the ability of the underlying trabecular bone todissipate forces through deformation of the subchon-dral plate. 6,7 The trabecular architecture of the proxi-mal femur also facilitates appropriate load transmis-sion through the formation of 3 distinct arcadesarranged at 60 orientations to manage the tensile andcompressive forces experienced by the femoral headand neck. The cortical structure of the femoral neck isthicker at the inferior margin as an additional adapta-tion to these loads. 8,9 The inherent stability afforded

by the depth of the acetabulum also denes the abso-lute limits of motion of the hip joint before the occur-rence of bony impingement. These limits occur inexion (120), extension (10), abduction (45), ad-duction (25), internal rotation (15), and externalrotation (35) 4 and may vary slightly between patients.

The articular surfaces are covered by multiplehighly organized layers of hyaline cartilage arrangedin a specic distribution to appropriately handle theforces placed across the hip joint. 10,11 The maximum

thickness is found at the ventral-cranial surface of theacetabulum and the ventrolateral surface of the fem-oral head with cartilage density decreasing concentri-cally from these points. 12 This cartilage consists of type II collagen and a high concentration of hydro-

philic glycosaminoglycans that trap water in the sub-stance of the cartilage and accentuate the stress-shield-ing properties of the joint surface. It functions tofurther absorb shock and dissipate the high forcesgenerated across the joint. This characteristic is syn-ergistic with the function of the subchondral bone toprovide a solid foundation for load transmissionthrough the hip. 13-15

The acetabular labrum is a complex structure con-sisting of a brocartilaginous rim composed of cir-cumferential collagen bers spanning the entirety of the acetabulum and becoming contiguous with thetransverse acetabular ligament. 16-18 The completephysiologic function of the labrum is not entirelydened, but it appears to serve multiple purposesincluding a limitation of extreme range of motion anddeepening the acetabulum to enhance the stability of the hip joint. The labrum contributes approximately22% of the articulating surface of the hip and in-creases the volume of the acetabulum by 33%. 4 Thisassists in dissipation of the large forces across the hipwith stride and athletic activities. 19 The labrum alsoprovides a sealing rim around the joint enabling in-creased hydrostatic uid pressure, to facilitate syno-vial lubrication and resistance to joint distraction. 20

Continuity with the transverse acetabular ligamentprovides an inherent elasticity that allows excellentconformity with the articular surfaces while compen-sating for minor joint incongruities. This allows thelabrum to function in its most important role of dis-sipating the high potential contact forces encounteredby the hip joint during activity and weight bearing atany exion angle. Recent surgical techniques havefocused on preservation and repair of the acetabularlabrum to maintain the intra-articular environment andminimize potential degenerative changes of the hip.

The dynamic stability of the hip is further aug-

mented by the strong surrounding capsule and liga-ments. The capsule is divided into 3 distinct bands thatfunction as external restraints to extreme motion. Themedial iliofemoral ligament, or Y ligament of Big-elow, originates from the area between the anteriorinferior iliac spine and the acetabular rim and insertsalong the anterior portion of the intertrochanteric line.Its role is to limit extension and external rotation of the hip, and it assists in the maintenance of a staticerect posture with minimal muscular activity. 21-23 The

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ischiofemoral ligament originates from the ischial rimof the acetabulum and follows the iliofemoral liga-ment as it twists around the femoral head and insertsonto the posterior aspect of the femoral neck, limitinginternal rotation and hip adduction with exion. The

femoral arcuate ligament is conuent with the poste-rior hip capsule and tensions the capsular tissue withextreme range of motion. 21 These ligamentous bandsbecome conuent with the capsule and further accen-tuate the static and dynamic stability of the hip joint.Biomechanical analysis has concluded that the il-iofemoral ligament is the strongest of the 3, able towithstand the highest amount of force before failureand affording appropriate stability against anteriortranslation and instability of the hip. 22,24 The stabiliz-ing role of the ligamentum teres is questionable be-cause it does not appear to contribute a signicantamount of restraint to the femoral head when com-pared with the capsular ligaments and the osseousanatomy. This structure does attain a state of mildtension during extreme hip adduction but only appearsto function as a secondary contributor to hip stability. 21

In the clinical setting, knowledge of the anatomiccomponents of the hip joint and their individual con-tribution to the architecture and stability of the joint,in combination with the history and physical exami-nation, helps the treating physician in identifying andevaluating the source of hip complaints. Given thecomplexity of the hip anatomy and varied clinicalpresentation of intra-articular pathology, this remainsa clinical challenge. 25 After identication of potentialsources of pathology, further diagnostic testing andtreatment including diagnostic injection and magneticresonance arthrogram can be used for further clinicalassessment. 26 This may help the orthopaedic surgeontarget the individual pathologies responsible for thepatients symptoms and appropriately direct care. Un-derstanding the potential future implications of injuryto the hip and possible treatment effects can also helpin predicting the development of recurrent symptomsor osteoarthritis.

GENERAL BIOMECHANICS

The femoroacetabular joint is unique in the fact thatit is never fully unloaded during daily activities. Al-though the duration of maximum loads experienced bythe articular surfaces of the hip may be short, there isa residual compressive force acting across the joint atall times, with an average magnitude approximatelyequal to the body weight. 27 Pauwels 28 dened theforces acting around the hip and the moments required

to balance the pelvis. The joint reactive force is thecompressive force experienced at the femoroacetabu-lar articulation, and it is the result of the need tobalance the moment arms of the body weight with thepull of the hip abductors at the greater trochanter to

maintain a level pelvis. The primary contributions tothe joint reactive force are the muscular forces gener-ated to level the pelvis during standing and gait, witha smaller contribution from body weight. 29 The mag-nitude of this force varies with activities such as thesingle-leg stance and phase of gait, and it has beenfound to be as much as 2 to 4 times the body weightduring level walking and stair ascent and slightlyhigher during stair descent. Because of the geometricoffset and anteversion of the proximal femur, a torqueis also applied to the femoral neck during these activ-ities, which must be tolerated by the structure of thebone and cartilaginous tissues. 30,31 Athletic activitiesmay greatly increase the magnitude of these forcesand place their orientations at the limits of the artic-ulation, requiring adjacent muscular, ligamentous, andcartilaginous structures to assist with load transfer.Normal gait takes the hip through a 40 to 50 arc of rotation, 35 of maximum hip exion, and 10 of maximum extension. 32 Smooth gait relies on a well-synchronized series of concentric and eccentric mus-cular contractions to facilitate a balanced stride. Acomplex neuromuscular loop exists that maintains theappropriate position between the femoral head andacetabulum with balanced muscular regulationachieved at both the voluntary and involuntary level.Proprioceptive feedback is provided both from theposition of the body and receptors in the hip capsuleand from intrinsic muscular properties, such as musclespindle ber and sarcomere length. 33 The magnitudesof the forces experienced in the hip during stride arebiphasic, with the force across the acetabulum reach-ing a maximum at heel strike and during terminalstance of the gait cycle. 34 These forces have beencalculated to be higher during an unassisted slow gaitwhen compared with a faster pace because of theabduction force generated by the gluteus medius and

minimus to maintain pelvis height during the pro-longed single-leg stance phase. 29 An association hasbeen found between being overweight and increasedpeak hip moments that may independently increasethe risk of lower-extremity injury and dysfunction. 35

The weight-bearing portion of the hip has beenfound to vary with position of the femur in relation tothe acetabulum and the amount of load placed throughthe articulation. During normal loading of a nonar-thritic joint during activities such as walking, a sig-

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nicant majority of the articular surface participates inweight bearing. This involves the anterior, superior,and posterior parts of the femoral head and forms 2columns of force that are transmitted within the ace-tabular margin, joining at the superior aspect of the

acetabular fossa.34

A band of articular cartilage in thefoveal region and on the inferior aspect of the femoralhead remains unloaded, whereas the peripheral artic-ular surfaces are loaded at the limits of joint motionincluding the acetabular margin and the labrum. 36 Theforces experienced by the proximal femur are trans-mitted through the combination of tensile and com-pressive trabeculae observed radiographically in a di-rection parallel with the long axis of the femoralneck. 17,37 The amount of tensile and compressive tra-beculae varies with the neck-shaft angle of the femur,with a valgus femoral orientation relying more heavilyon compressive trabeculae for transference of load anda varus alignment relying more heavily on the tensilearcades. 28 The geometric orientation of the articularcartilage is also optimized for load transfer, becausethe thickest portions are at the areas of the acetabulumand femoral head most frequently loaded duringgait. 12 During repetitive hip motion, the vector of the joint force rapidly uctuates, and a mismatch in thestructural properties of the joint may be encountered.This has been hypothesized to predispose the hip tofrequently observed patterns of injury or degenerationbecause the compressive abilities of the articular car-tilage vary according to their location. 13-15,36

These general principles of hip biomechanics havesignicant clinical relevance with regard to the nativefunction of the hip joint in the absence of pathologyand must be considered when one is evaluating apatient. Many factors contribute to the forces encoun-tered in the hip, including daily and athletic activities,the contribution of weight and obesity, and the lim-itations of femoroacetabular motion. Rehabilitationafter injury or surgical intervention of the hip mustalso respect these principles to restore function andminimize further pathologic or degenerative change.

PATHOLOGIC BIOMECHANICS ANDSURGICAL INTERVENTION

Biomechanics in the development or as a result of pathologic conditions in the hip may result from an-atomic alteration, congenital deciency, injury, or de-generation. Familiarity with the biomechanical causesof various pathologies of the hip and the consequencesof anatomic variations of the structures comprising the

joint allows the orthopaedist to recommend appropri-ate treatment.

Osseous Anatomy

Alteration of the inherent osseous stability of thehip can have signicant consequences on the forcesand contact areas experienced at the joint surface. Thisis clearly shown in the evaluation of the force transferacross the hip in the setting of hip dysplasia, coxavara, and coxa valga. In dysplastic conditions in whichthere is insufcient coverage of the femoral head bythe acetabulum, the contact between the articular sur-faces is concentrated on a small area of articularcartilage on the lateral aspect of the acetabulum. Ca-daveric studies have shown that these contact forcescan be as high as 260% of the body weight during thesingle-leg stance. 38 This focal area of increased con-tact forces has been implicated in the clinical devel-opment of early hip degeneration and painful arthritis.Because of the lack of osseous coverage for the fem-oral head, the labrum has also been found to becomehypertrophied superiorly and may participate in pro-viding load transfer. 39 Debridement or reduction of the hypertrophied labrum without addressing the ace-tabular dysplasia can result in migration of the femoralhead out of the acetabulum and the potential to de-velop accelerated degenerative changes. 40 Coxa valgaplaces the abductor muscles in a less ideal position bymedializing the trochanter with respect to the center of rotation of the femoral head, increasing their requiredpull to maintain the pelvis at a level state and therebyincreasing the joint reactive force. Coxa valga in com-bination with insufcient acetabular coverage createsa large contact force concentrated on a narrow band of articular surface on the lateral edge of the acetabulum,potentially leading to early symptomatic osteoarthro-sis. 31 Coxa vara, in contrast, actually places the ab-ductor muscles in a more advantageous location tomaintain the pelvis at a level state while allowingincreased coverage of the femoral head and articularcongruity. Imbalance of the weight-bearing axis or

muscular pull in the setting of coxa vara can lead toincreased contact stress on the medial articular carti-lage and medial migration of the femoral head, lead-ing to acetabular wear and protrusion. 31

Surgical management for correction of osseousanatomy to correct or optimize the biomechanics of the hip can be performed on the acetabulum, theproximal femur, or both. Pelvic osteotomy is a pow-erful tool allowing reorientation of the hip articulationwith a change in the morphology of the acetabulum.



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The periacetabular and Salter osteotomies improve theanterior, lateral, and superior coverage of the femoralhead in the condition of developmental dysplasia of the hip and have been shown in cadaveric studies todecrease the contact force across the articular cartilagefrom up to 270% of the body weight to less than 120%of the body weight. These also have the advantage of increasing the joint surface area across which thecontact force is distributed while optimizing the me-chanical advantage of the abductor musculature anddecreasing the force necessary to maintain pelvic bal-ance. 38,41 Intertrochanteric osteotomy is another pow-erful tool that can be used to redirect the femoral headinto the acetabulum, optimizing the contact surfacesbetween the joint, centering the vertical joint reactiveforce well within the dome of the acetabulum, and

redirecting the muscular balance of the gluteus mediusand minimus. 31

Femoroacetabular Impingement

FAI is a condition that results in abnormal contactbetween the bone of the proximal femur and theacetabulum due to alteration of the osseous morphol-ogy of the hip. This creates a force on the acetabularlabrum producing injury, pain, and tearing that caninitiate a cascade of chondral injury and potentialdegenerative changes. Two distinct types of FAI havebeen described in the literature, cam type and pincer

type. Cam-type FAI results from decreased offset be-tween the femoral head-neck junction, leading to im-pingement of a prominence on the femoral neck against the acetabular rim during specic hip motionssuch as exion, adduction, and internal rotation. Thiscontact generates an outside-in abrasion/compressionof the acetabular labrum, resulting in tearing or avul-sion of the cartilaginous tissue from its origin at theacetabular rim. Pincer impingement results from lin-ear contact between the acetabular rim and the femoral

head-neck junction due to abnormalities of the ace-tabular morphology. These abnormalities include ret-roversion of the acetabulum, coxa profunda, and in-creased anterior and superior acetabular coverage 42

(Fig 1 ). FAI creates a scenario in which the acetabularlabrum is vulnerable to both acute and chronic injuriesthat can lead to symptomatic hip pain and degenera-tive changes in the labral and articular tissues.

As understanding of the function of the labrum inmaintaining the stability of the hip and protecting thearticular cartilage increases, attention has been placedon surgical techniques that aim to restore these func-tions through repair of the labral tissue. Integral to therestoration of labral function are identifying and ad-dressing the underlying cause of the labral injury.Current techniques of hip arthroscopy allow mini-mally invasive evaluation of the articular surface of the hip and the acetabular labrum. The presence of cam- or pincer-type FAI can also be evaluated andmanaged concomitantly with labral and articular car-tilage pathology. A selective femoral neck osteoplastycan be effectively performed under arthroscopic guid-ance to remove any osseous impingement from thefemoral head-neck junction ( Fig 2 and Videos 1-6[available at www.arthroscopyjournal.org ]). Manage-ment of pincer-type FAI is more complicated andinvolves elevation of the labrum from its insertion onthe acetabular margin followed by debridement of theunderlying bone to correct the acetabular morphologyand relieve the compression placed on the labrum.Care must be taken to avoid causing an iatrogenic lossof femoral head coverage through excessive resectionof acetabular bone while attempting to manage pincer-type FAI, potentially leading to increased loads acrossthe articular surface, articular cartilage damage, andsubluxation or migration of the femoral head out of the acetabulum. 24,43

F IGURE 1. (A) Cam-type FAI showing decreased offset at femoral head-neck junction. (B) The femoral head impinges against theanterosuperior labrum in the position of hip exion, adduction, and internal rotation. (C) Pincer-type FAI showing increased acetabularcoverage. (D) In exion and internal rotation, the acetabular margin abuts the anterior femoral neck and impinges on the anterior labrum.(Reprinted with permission from Leunig et al. 65 )

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F IGURE 2. (A) Arthroscopic image of a large prominence on the anterior femoral neck (arrows) in a right hip through a standard anterolateralportal causing impingement against the acetabular labrum consistent with cam-type FAI. (B) A selective femoral neck osteoplasty wasperformed with complete removal of the bony prominence. (C) The anterior femoral neck was checked in exion, adduction, and internalrotation to conrm arthroscopically that the impingement against the labral rim had been resolved. (D) Preoperative radiograph showing theprominence on the superolateral portion of the femoral neck (arrow). (E) A postoperative lateral radiograph of the same hip shows the femoralneck osteoplasty and removal (arrow) of the previously identied prominence.



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Labral Injury

The acetabular labrum, or injury thereof, has alsobeen implicated as a cause of osteoarthritis of thehip. 44 Studies have shown that the absence of thelabrum may increase the rate of articular cartilageconsolidation by 40%, with associated increases incontact stress in the acetabular cartilage by as much as92%. 45 Absence of the labrum transfers the contactarea of the femoroacetabular cartilage laterally towardthe acetabular margin, with associated translationalmotion of the femoral head within the articulation, 45

and signicantly reduces resistance to distraction of the joint surfaces. 20 Cadaveric studies have failed toreproduce all of these ndings, but the altered loadingand biomechanical function of the hip with increasedcontact stresses and lateralization of the contact sur-face may potentially play a role in the development of osteoarthritis. 16,40 A signicant association betweenthe presence of labral lesions and degenerativechanges of the articular cartilage of the femoral headand acetabulum has been observed arthroscopically,with up to 74% of patients with labral fraying ortearing of the labrum having identiable chondralinjury. These tears and associated lesions occur in thesame region of the articular surface in 80% of patients,with the strongest associations occurring both poste-riorly and laterally. 40 These ndings have also beenconrmed in cadaveric studies, supporting the ideathat labral tears and joint disease are part of a progres-

sion of joint pathology. 46After concurrent hip pathology has been evaluated

and managed, treatment options for management of labral tears include debridement, repair, and recon-struction. Tears that are not repairable include fraying,radial tears and degenerative tears in which the bloodsupply is not amenable for healing or the disruption of the longitudinal bers prohibits adequate repair. Thegoal of labral debridement is to create a stable baseand minimize the discomfort associated with unstableap tears ( Fig 3 ).

Primary labral repair is appropriate in longitudinal

tears of the labrum that do not signicantly violate thelongitudinal bers of the structure or in the case of avulsion-type injuries of the labral base from the ac-etabular margin. Intrasubstance splits may also beamenable to primary repair if the base has remainedwell xed to the acetabular rim and there exists astable outer rim. 47 Repair generally involves place-ment of suture anchors along the capsular margin of the acetabulum and reapproximation of the labrum tothe acetabular rim through the use of arthroscopically

tied knots in an attempt to restore the native functionof the tissue. 48 Recently published studies evaluatingthe management of labral tears associated with FAIhave shown signicantly improved outcomes afterlabral repair versus debridement. 49,50 These early, im-proved clinical outcomes after labral repair in thesetting of FAI are encouraging, and future investiga-tions may prove the benet of labral preservationsurgery in the delay or alteration of the natural courseof degenerative hip arthritis.

F IGURE 3. (A) Arthroscopic view of a labral tear in the left hipfrom a standard anterolateral viewing portal. There is signicantfraying of the labral substance and multiple planes of the tear. (B)This was managed with debridement to a stable rim while preserv-ing as much labral tissue as possible. There is associated peripheralarticular cartilage damage visualized on the adjacent femoral head.Acetabular rim trimming was concomitantly performed because of pincher impingement.



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Capsular Laxity and Hip Instability

The concept of hip instability and capsular laxityhas recently emerged as an identiable and potentiallycorrectable cause of hip pain and disability. 2 The

origin of hip instability can be divided into traumaticand atraumatic causes, with traumatic hip instability

usually resulting from a clearly dened event of hipsubluxation or dislocation. This may be associatedwith a high-energy trauma such as a motor vehicleaccident or from a low-energy injury that occursmore commonly during athletic activities. 51,52

These injuries may be associated with bony injuriesto the femoral head or acetabular wall or withshearing injuries to the articular cartilage with acompromise in the load-transferring ability of the joint. The onset of atraumatic hip instability is lessdistinct and may be due to repetitive microtrauma,generalized ligamentous laxity, iatrogenic causes,and connective tissue disorders. 24,53 It has beenhypothesized that atraumatic instability may be theresult of repeated injury to the ligamentous capsuleduring activities that force the hip into abductionand external rotation. These positions increase the

forces in the iliofemoral ligament, resulting in the de-velopment of capsular laxity and predisposing theacetabular labrum to injury. Once the static stabilizersof the hip including the capsule and labrum are com-promised, there is an increased reliance on the dy-namic stabilizers of the hip during activity, with thedevelopment of overuse syndromes and associatedsymptoms of the surrounding musculature ( Fig 4 ).54

Subclinical instability associated with capsular laxitymay also be an underlying cause of painful coxasaltans, or snapping hip. Increased mobility of the hipmay allow the iliopsoas tendon to glide abnormally

over the proximal femur and pubic ramus or the ili-otibial band to snap over the greater trochanter,

F IGURE 4. Fluoroscopic images of a patient lying supine on astandard operating room table after traction has been applied to theleft hip. There is signicant opening of the femoroacetabular jointwith minimal distracting force before insertion of a needle orcannula consistent with increased capsular laxity.

F IGURE 5. Capsular laxity and in-stability of the hip may result insymptoms of hip pain through sec-ondary impingement or through al-tered loading on the intra-articularstructures. Surgical managementincludes arthroscopic capsular pla-cation as diagrammed. This in-volves performing an arthroscopicanterior hip capsulotomy with ad-vancement of the lateral and me-dial arms of the iliofemoral liga-ment to reduce capsular volume inan attempt to restore hip stability.(Reprinted with permission fromTibor and Sekiya. 2 )



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resulting in clinical symptoms of painful and some-times audible snapping in the hip with provocativemaneuvers. 55

Instability of the hip results in an excessive amountof translational motion at the femoroacetabular artic-

ulation in addition to the rotational motion normallyexperienced at the articular surface. This aberranttranslational motion changes the dynamic loading atthe articular surface, creating a mismatch in the artic-ular cartilage orientation and potentially leading toearly cartilage wear and degenerative change. 13 Theincreased translation of the femoral head also placesthe acetabular labrum at risk of shear injury and re-petitive microtrauma, further compromising the jointand contributing to pathologic changes within thehip. 42

Surgical management for capsular laxity has in-cluded plication with a suture technique versus ther-mal capsulorrhaphy. 53 The goal of these is to increaseor restore the pre-elongation length of the iliofemoralligament, as well as to reduce the overall volume of the capsular complex 42 (Figs 5 and 6). This potentiallydecreases the translational motion of the femoroac-etabular joint while protecting the labrum from in-creased shear forces associated with this aberrant mo-tion. There have not been any formal biomechanicalstudies to evaluate the effects of capsular plication of thermal shrinkage on the stability of the hip, butclinical outcomes appear to be favorable in success-fully decreasing the preoperative symptoms of hipinstability when performed in conjunction with surgi-cal management of concomitant hip pathology. 42

Articular Cartilage

The ultimate consequence of biomechanical chan-ges of the hip joint results in an alteration of thearticular cartilage leading to degenerative change oracute injury. The goal in the surgical management of hip pathology is to decrease the symptoms of hip painwhile preserving the articular cartilage because anydefects rarely heal spontaneously, whether caused by

acute, chronic, or degenerative injury.56

After the de-velopment of articular cartilage injury, it can be verydifcult to restore the native function of the joint, andsuch injury usually results in progressive degenerativechanges leading to symptomatic osteoarthritis. Focalchondral defects may be due to a direct-blow injury ordelamination as a result of FAI and labral injury.Acute causes have frequently been attributed to alateral-blow mechanism at the greater trochanter.Given the subcutaneous location of the trochanter, the

F IGURE 6. Arthroscopic visualization of the peripheral com-partment in a right hip through a standard anterolateral portal.(A) After a selective femoral neck osteoplasty, the capsulotomyperformed to gain access to the hip joint is prepared for capsularplication. (B) Sutures are shuttled through the capsular tissueand placed in a manner to allow advancement of the individuallimbs of the capsulotomy. (C) The tissue is then arthroscopi-cally oversewn to decrease the intra-articular volume and re-store stability of the hip.



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impact force is directly transferred through the densecortical bone to the joint surface, resulting in chondrallesions of the femoral head or acetabular surface.Arthroscopic ndings of focal defects after such aninjury commonly support this lateral-impact mecha-

nism.57

Chondral lesions are also frequently associ-ated with labral lesions, with some clinical seriesreporting up to a 72% correlation with arthroscopi-cally diagnosed labral injury and concomitant chon-dral lesions, suggesting that labral injury and jointinjury are a continuum in the development of degen-erative osteoarthritis. 46

Methods of surgical treatment to restore the arti-cular cartilage include microfracture, primary repair,autologous cartilage transplantation, osteoarticular au-tograft, focal arthroplasty, and total hip arthropla-sty. 43,58 Controversy exists about which option is moreappropriate in the individual patient, underlining thefact that there is no consensus on the optimal strategyto preserve or restore the articular surface. Microfrac-ture has been advocated in treating full-thicknesschondral defects of the articular surface measuringbetween 2 and 4 cm in size. This may be an appro-priate conservative option in select patients to delay orpossibly prevent the need for total hip arthroplasty 59

(Fig 7 ). The tissue that is stimulated with microfrac-ture consists mainly of brocartilage, which has sig-nicantly different characteristics than those of nativehyaline articular cartilage but nonetheless is a poten-tial improvement over the exposed subchondral bonefound in full-thickness osteochondral lesions. 60 Pri-mary repair of large delaminated lesions of the artic-ular cartilage with a suture technique has been re-ported in young patients in whom other options wouldbe less optimal. The early clinical results of this tech-nique appear favorable in appropriately selected pa-tients. 43 The use of autologous chondrocyte implanta-tion for treatment of osteochondral lesions of the hip isexperimental at this time, with limited case reportsshowing moderate outcomes. 61 Mosaicplasty ap-plies the idea of harvesting autogenous osteochondralgrafts from nonweight-bearing portions of adjacent

joints and transplanting them into a focal cartilagedefect in an attempt to restore the integrity of thearticular surface. Multiple case reports exist regardingthe use of this technique for salvage of major chondrallesions of the hip associated with trauma or avascularnecrosis. 62 Osteochondral allograft reconstruction of the proximal femur has also been described, withsome series reporting clinical success with this tech-nique as an intermediate option in young patients withfemoral head collapse due to avascular necrosis. 63

F IGURE 7. (A) Focal chondral defect of the femoral head viewedarthroscopically through a standard anterolateral portal. The mar-gins of the lesion are stable to probing with no underlying articulardelamination. (B) The lesion was debrided to subchondral bone,and (C) a microfracture technique was performed through anaccessory lateral portal to help restore the articular surface withbrocartilage.



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Partial resurfacing hemiarthroplasty for focal chondraldefects of the hip has been described in the literature,but clinical or outcomes data on this procedure arelimited and it cannot be recommended at this time forroutine care of hip lesions. 64 The clinical results of

these various techniques have been modest at best butdo represent options for patients with signicant ar-ticular surface injury of the hip who are not appropri-ate candidates for total hip arthroplasty.

The ideal management of these difcult problems isprevention of developing articular cartilage lesionsthrough the appropriate use of both conservative andsurgical measures aimed at restoring the native bio-mechanics, kinematics, and biology of the hip. Under-standing the biomechanical consequences of patho-logic conditions allows the clinician to implement anddevelop current surgical techniques that make sense inaltering the course of diseases of the hip. This under-standing can be readily applied to improving bothcurrent and future patient care in the management of these difcult and complex conditions.

CONCLUSIONS

The hip is a complex anatomic structure composedof osseous, ligamentous, and muscular structures re-sponsible for transferring the weight of the body fromthe axial skeleton into the lower extremities. Thismust be accomplished while allowing for dynamicloading during activities such as gait and balance.Given this complex interplay between structures, theevaluation of hip pain can be technically difcultbecause of the multiples causes that may be respon-sible for similar symptoms of hip pain. A detailedunderstanding of the complex anatomy and biome-chanics of the hip in conjunction with a focused phys-ical examination, diagnostic injection, and appropriateradiographic studies can help the orthopaedic surgeonto successfully diagnose and treat complex patholo-gies of the hip. 26

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Biomechanics of the Hip

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