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..

Force◦ definition

a push or pull on an object resulting from the object's interaction with another object

◦ equation force = mass x acceleration, F=ma 1 Newton = force required to give 1 kg mass an

acceleration of 1 m/s2 

Vector◦ definition

a quantity that contains both direction and magnitude

scalar quantities do not have direction ◦ forces and velocity can be broken down into

vectors

Moment (torque)◦ definition

the tendency of a force to rotate a body around an axis

◦ Equation◦moment (torque) = force(perpendicular) X

distance

Work◦ definition

when a force acts upon an object to create displacement

◦ equation work = force (vector parallel to displacement) x

distance

Newtons laws◦ first law

if there is no net force on an object, its velocity remains constant

◦ second law force equals mass multiplied by acceleration F=ma

◦ third law when 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 body

F2=-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 joint◦ in the hip, it is the result of the need to balance the moment arms of the body

weight and abductor tension (see diagram to right)◦ maintains a level pelvis

Coupled forces◦ when two movements and associated forces are coupled

Joint congruence◦ relates to fit of two articular surfaces◦ high congruence increases joint contact area

Instant center of rotation◦ point about which a joint rotates◦ often changes during rotation due to joint translation◦ center of gravity of human is just anterior to S2

Friction and lubrication◦ not a function of contact area◦ lubrication decreases friction◦ examples

coefficient of friction of human joints is .002 to .04 TJA (metal on PE) is .05 to .15

What all matters in a joint…

Hip Free Body Analysis

Actions that decrease joint reaction force include ◦ increase in ratio of A/B (shift center of rotation medially)

moving the acetabular component as far medial, inferior, and anterior  shifting body weight over affected hip  

this results in Trendelenburg gait . increasing offset of femoral component  long stem prosthesis lateralization of greater trochanter varus neck-shaft angulation

increases shear across joint◦ cane in contralateral hand ◦ reduces abductor muscle pull and decreases the moment arm

between the center of gravity and the femoral head  carrying load in ipsilateral hand 

produces additional downward moment on same side of rotational point Actions that increase joint reaction force include

◦ valgus neck-shaft angulation 

decreases shear across joint

Clinical implications…

Coxa vara and coxa valga..

APPLIED BIOMECHANICS

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:

1. • Range of motion 2. • Impingement implant fixation 3. • Tissue damage during implantation and

tissue tension after THA4. • Component orientation (stem, cup) 5. • 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.

Disadvantages1. higher friction moments(especially MOM bearings).2. Requires better fixation of bearing other

componenets . 3. Cup loosening and Corrosion4. More dissection required –more soft tissue damage

during primary surgery or during relocating a large head dislocation.

A larger head..

Head size Dislocation rates

22mm Upto 18 %28mm 0.6-3%32mm 0.5%38mm 0.0%

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 patient’s 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 on

Tissue 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 plane

Cup 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.

To avoid impingement, a cup inclination of 45° to 55° has been recommended, the optimal combined anteversion depending on the acetabular abduction angle.

According to Yoshimine et al , a cup position avoiding impingement can be estimated with the following formula :

(Cup inclination) + (Cup anteversion) + 0.77× (Stem anteversion)) = 84.4.

(Stem anteversion) = (37° - Cup anteversion)/0.7.

From a clinical point of view, a “safe zone to avoid hip dislocations” has been defined between 40° ± 10° of cup abduction and 15° ± 10° or 30° ± 10° of anteversion.

To avoid excessive wear, the cup abduction angle should be 45° or less .Steep cups, with an abduction angle of 50°-55° or more, are prone to excessive wear and edge loading especially with metal-on-metal articulations and when microlaterization or microseparation do occur..not much of an issue with ceramic heads..

Effect of cup postiotion on wear..

HORIZONATAL CUP VERTICAL CUPIncreases ant sup coveragePreffered to reduce wear while walking.Uncovers post-inf acetabulamReduce cup loading area in deep flexion(rising from chair)Increase risk of dislocation in flexion,adduction,internal roationespecially when a posterior approach has weakened the soft tissues.

As a more vertical cup position tends to decrease the anterosuperior contact area of the cup, Edge loading

Increase postero-inferior coverage

As a more vertical cup position tends to decrease the anterosuperior contact area of the cup, this should be compensated for by decreasing cup anteversion. Opposite, a horizontal cup should be implanted with more anteversion.

Effect of decreasing cup coverage..

So, lowering the cup coverage to limit impingement decreases the boundaries of an acceptable cup position in terms of wear.

Additionally, lowering cup coverage decreases the press fit of the cup in the acetabular bone and the implant/bone contact area, making these cups more vulnerable to early loosening.

Accepting such compromises to allow a better ROM only makes sense for hip resurfacings because of the unfavourable head/neck ratio.

Acta Orthop. Belgium., 2014, 80, 336-347 The Open Sports Medicine Journal,2010, 4, 51-

57 Damien P. Byrne, Kevin J. Mulhall and Joseph F. Baker Orthopaedic Research and Innovation Foundation, Sports Surgery Clinic, Santry, Dublin, Ireland.

Atlas of Human Anatomy, Sixth Edition- Frank H. Netter, M.D

Apley’s System of Orthopaedics and Fractures 9th Ed

Campbell's Operative Orthopaedics 12th

Thank you…