biomechanics of the hip joint

anatomy

Acetabulum

· hemispherical

· cavity faces

· forward

· laterally

· downward

Femoral neck

· neck-shaft angle is 125^o

· anteversion angle is 12^o

Kinematics

Range of movement

· large range in all three planes

· maximised by

· ball and socket configuration of joint

· off-set of femoral neck

Range

· sagittal plane

· 30^o extension and 140^o flexion

· coronal plane

· 30^o adduction and 50^o abduction

· transverse plane

· 45^o internal rotation and 45^o external rotation in flexion

· rotation less in extension because of soft tissues

Instant centre

· instant centre unable to be analysed

· all movement is rotation about centre of femoral head (spin)

Gait

Sagittal plane

· maximal flexion during late swing phase

· when limb moved forward for heel strike

· reaches 40^o

· maximal extension at heel off

· only 5^o

Coronal plane

· abduction during swing phase

· adduction during stance phase

· range is 12^o

Transverse plane

· external rotation during swing phase

· internal rotation during stance phase

· range is 12^o

Daily activities

· ROM required for daily activities normally is

· 120^o flexion

· 20^o abduction

· 20^o external rotation

Kinetics

· in one legged stance, joint reaction force is 3 - 6 x body weight

· primarily due to contraction of abductors

· calculated with free body diagram

· assume

· sum of moments = S MR

· W = body weight

· weight of one leg is 1/6 body weight

· M = abductor muscle force

· M acts at 30^o from vertical

· R = reaction force of joint

· lever arm from centre of rotation of hip to gluteus medius tendon is A = 5 cm

· lever arm from centre of rotation of hip to centre of gravity is B = 12.5 cm

· calculation is

· S MR = 0

· 5/6 x W x 12.5 = M x 5

· M = 2.1 W

· My = M x cos 30^o

· My = 1.8 W

· S Fy = 0

· - My + -(5/6 x W) + Ry = 0

· Ry = 2.6 W

· R = Ry / cos 60^o

· R = 3 W

· increase in ratio of A : B

· caused by medialisation of acetabulum or increased offset of femoral neck

· causes decreased joint reaction force

· decrease in ratio of A : B

· caused by coxa valga

· causes increased joint reaction force

· antalgic stance is where body weight placed over affected hip

· moves centre of gravity towards centre of rotation

· effectively decreases B and increases A : B

· causes decreased joint reaction force

dynamics

Gait

· loads on hip during walking are maximal during stance phase

· 4 W just after heel strike (2.5 W in women)

· 7 W just before toe off (4 W in women)

· are less (about body weight) during swing phase

· increase in velocity increase magnitude of force in both phases

Other activities

· about 4 W acts on hip joint when patient uses elbows and heels and elevates on bed pan

· hip spice reduces forces by 2/3 for all activities

Cane

· cane should be used on opposite side to pathology

· reduces force on femoral head without antalgic stance

· reduces amount of contraction required by abductor muscles

· long lever arm to cane means moderate force significantly counteracts body weight