Biomechanics of hip

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Biomechanics of hip and knee

Biomechanics of hip MODERATOR:- DR PAVAN CHEBBI (ASSISTANT PROFESSOR SGITO)PRESENTER:-DR ABHISHEK CHUADHARY (DNB RESIODENT AT SGITO)

It can be defined as the science concerned with the internal and external forces acting on the human body and the effects produced by these forces.

Julius Wolff addressed the relation between the inner architecture of the bone and the functional loading already in the nineteenth century.

Friedrich Pauwels built the foundation for a mechanical approach to understand joint loading 65 years later.

Biomechanics

Hip-Bony anatomy

ACETABULAM innominate bone with contributions from the ilium (approximately 40% of the acetabulum), ischium (40%) and the pubis (20%) .

fusion of this starts to occur around the age of 14 16 years and is complete usually by the age of 23.

A normal gait

Normal ROM at hip..

Forcedefinitiona push or pull on an object resulting from the object's interaction with another objectequationforce = mass x acceleration, F=ma1 Newton = force required to give 1 kg mass an acceleration of 1 m/s2

Vectordefinitiona quantity that contains both direction and magnitudescalar quantities do not have directionforces and velocity can be broken down into vectors

Moment (torque)definitionthe tendency of a force to rotate a body around an axisEquationmoment (torque) = force(perpendicular) X distance

Workdefinitionwhen a force acts upon an object to create displacementequationwork = force (vector parallel to displacement) x distance

Newtons lawsfirst lawif there is no net force on an object, its velocity remains constantsecond lawforce equals mass multiplied by accelerationF=mathird lawwhen a first body exerts a force on a second body, the second body exerts a force that is equal in magnitude and opposite in direction on the first bodyF2=-F1

The hip is a true ball-and-socket joint surrounded by powerful and well-balanced muscles, enabling a wide range of motion in several physical planes while also exhibiting remarkable stability.Hip..

Joint reaction force defined as force generated within a joint in response to forces acting on the jointin the hip, it is the result of the need tobalance the moment armsof the body weight and abductor tension (see diagram to right)maintains alevel pelvisCoupled forceswhen two movements and associated forces are coupledJoint congruencerelates to fit of two articular surfaceshigh congruence increases joint contact areaInstant center of rotationpoint about which a joint rotatesoften changes during rotation due to joint translationcenter of gravity of human is justanterior to S2Friction and lubricationnot a function of contact arealubrication decreases frictionexamplescoefficient of friction ofhuman joints is .002 to .04TJA (metal on PE)is .05 to .15What all matters in a joint

Hip Free Body Analysis

Actions thatdecrease joint reaction forceincludeincrease in ratio of A/B (shift center of rotation medially)moving the acetabular component as far medial, inferior, and anteriorshifting body weight over affected hipthis results in Trendelenburg gait.increasing offset of femoral componentlong stem prosthesislateralization of greater trochantervarusneck-shaft angulationincreases shear across jointcanein contralateral hand

reduces abductor muscle pull and decreases the moment arm between the center of gravity and the femoral headcarrying load in ipsilateral handproduces additional downward moment on same side of rotational pointActions thatincrease joint reaction forceincludevalgusneck-shaft angulation

decreases shear across joint

Clinical implications

Coxa vara and coxa valga..

APPLIED BIOMECHANICS

STANCE PHASE 3 BW,,,,,,,IN ARTHRITIS DDH EXT ROTN DEFORMITYLEVER ARM IS SHORTENED30

The location of center of rotation of femoral head is determined by

1. Vertical offset2. Horizontal(medial) offset3. Anterior offset (Anteversion)

So restoration of these dimentions is must

MEDIAL RESTORATION IS SIMPLY CORRECTED BY MAKING NECK ADJUSTMENT BUT

LIMB LENGTH INCREASES

NORMAL FEMUR IS 10 TO 15 DEGREE ANTEVERTED.USUALLY ACCOMPLISHED BY ROTATING THE COMPONENT IN FEMORAL CANAL.IN PRESS FIT FIXATION IS USED MODULAR FEMORAL COMPONENT IS USED.VERSION

AFFECTS ROM ,IMPINGEMENT,STABILITY OF ARTICULATION.

HEAD NECK RATIO

Its complicated ..

Because all joint parameters are influenced by the operation: joint center, neck angle, offset, lever arms, and the range of motion until impingement.

Our goal is best possible range of motions with good stability and prolong implant life .Biomechanics of THR

design and implantation procedure, which directly or indirectly influence the occurrence of loosening, lysis, or dislocation. These aspects are: Range of motion Impingement implant fixation Tissue damage during implantation and tissue tension after THA Component orientation (stem, cup) Bearing material

Factor influencing ROM

Biomechanically most important is the position of the femur with respect to the pelvis, in which the end of the RoM is reached and the prosthesis neck impinges on the cup or impingement occurs somewhere else between femur and pelvis. Impingement can lead to subluxation or even dislocation of the hip joint. If impingement occurs repeatedly in positions inside the RoM required by the patient for either daily or athletic activities, dislocation is rather probable. The jumping distance, which is the distance the head has to jump before leaving the cup, amounts in hemispherical cups to 50% of the head diameter.

Advantages 1.Increased ROM2.Increased jumping distance.

Disadvantageshigher friction moments(especially MOM bearings).Requires better fixation of bearing other componenets . Cup loosening and CorrosionMore dissection required more soft tissue damage during primary surgery or during relocating a large head dislocation. A larger head..

Head size Dislocation rates 22mmUpto 18 %28mm0.6-3%32mm0.5%38mm0.0%

So how large it has to be Size is not the only parameter for dislocation tho Implant position and soft tissue tension is equally important

Practically head diameter is limited to about 36 mm for primary hip arthroplasty; in the case of Polyethylene possibly even to 32 mm, since for hard-soft bearings wear increases with head diameter.Geometry of Taper..A further design aspect important for the technical RoM is the location of the entrance plane of the cup and the cup profile

Presently, cemented fixation still shows statistically the best results in terms of the whole THA population (Table 2.1).

This changes when young and active patients are involved; in this patient collective, cemented prostheses do not perform as well as in the older population

The success of uncemented fixation depends on the ingrowth of bone. The ingrowth of bone is only possible, if the patients activity-induced relative interface motion (micromotion) remains below a critical threshold in the early postoperative period (primary stability).So good quality reaming and press fitting followed by non/partial weight bearing for few weeks results in good bone ingrowth.

The amount of soft tissue damage during surgery and the tension in the remaining soft tissue after THA implantation are also important factors for the stability of the joint.

Hard tissue damage can also occur during implantation. This can either result in direct complete fractures or fissures of the bone, or in micro-fractures of trabecular bone, which can develop to a complete fracture later onTissue Damage and Joint Tensioning

Component orientation and position is probably the most important biomechanical aspect for the tribological and functional success of a THA procedure.

Component Orientation..

If the combined offset decreases, abductor muscle tension drops and hip instability can occur. Opposite, an increased combined offset can produce excessive tension within the gluteal muscles and the iliotibial band and cause trochanteric pain.

The combined offset..

Hip length..

The cup inclination angle..Without leg length discrepancy, the interteardrop line is perpendicular to the longitudinal axis of the patient, and the cup inclination angle equals the lateral opening angle.

The radiographic cup anteversion is defined as the angle between the acetabular axis and the coronal plane.Because the projection of the acetabulum depends on the pelvic tilt (i.e. the angle between the anterior pelvic plane and the coronal plane of the body), it is important to measure cup anteversion on standardized radiographs, i.e. with the radiographic film parallel to the anterior pelvic planeCup anteveresion

Cup and stem anteversion have opposite effects on impingement. Increasing cup anteversion and decreasing stem anteversion will favour impingement of the posterior aspect of the neck against the posteroinferior rim of the acetabulum in extension and exorotation . Opposite, decreasing cup anteversion and increasing stem anteversion will favour anterosuperior impingement in flexion and adduction.

Stem anteversion is difficult to control specially in uncemented ones.

Cup inclination influences the degree of anterosuperior and posteroinferior coverage, and so the impingementfree range of motion.

A horizontal cup position increases anterosuperior coverage, but uncovers the posteroinferior aspect of the acetabulum.

Opposite (more vertical cup), increasing the cup abduction angle will expose the anterosuperior aspect of the acetabulum but will increase the posteroinferior coverage.