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BIOMECHANICS OF HIP By : Dr . Belehalli Pavan INTRO AND HISTORY • Pauwel ( 1935 ) – First described the mechanical aspects of functioning of a hip joint. • Delineated the direction and magnitude of forces across the hip in normal and pathological conditions. BASIC DEFINITIONS • Biomechanics – Science of action of forces internal or external on the living body. • Dynamics – Study of motion of bodies and forces that produce the motion. Studied under 3 headings a) Kinematics – Study of motion in terms of displacement velocity and acceleration without ref to the cause of motion. DYNAMICS (contd) b) Kinetics – Relates to action of forces on bodies to their resultant action. c) Kinesiology – Study of human movement or motion. Def related to Biomechanics • FORCE - Push or pull causing int or ext effects - Can be split into their components in x and y axes. - Resultant force – Single force equivalent to a system of forces ; Calculated by adding all the forces acting at a point. - Equilibriant force – Force equal in magnitude and opposite in direction to Resultant force. Def (contd) • Moment (M)Rotational effect of a body about a point ; equlas Force perpendicular distance from a specified point (moment arm) M = F *d Def ( contd ) • Free Body Diagram ( FBD ) – Skeleton of a body or portion thereof isolated from all other bodies showing all forces acting on it. Defn ( Contd ) • Degree of freedom – Described based on rotation and translation in x , y and z axes. • Joint reaction force (R) – Forces generated within a joint in response to external forces. • Instant center of rotation – Point at which a joint rotates; normally lies on a line perpendicular to the tangent of the joint surface at all points of contact . Hip Biomechanics • Forces acting across a hip joint can be measured either ▫ Directly with implanted strain-gauged endoprosthesis ▫ Mathematical model calculations – 2D static analysis FBD ANALYSIS A B • R in the hip can reach three to six times body weight (BW) and is primarily due to contraction of muscles crossing the hip. This can be demonstrate with a FBD. • If A=5 and B=12.5, using standard FBD analysis, Joint Reaction Force R = 4W. Analysis (Cntd) • By the fig we can see that the R can be decreased by • 1. Medialisation of Acetabulum • 2. Long neck Prosthesis • 3. Lateralization of Greater Trochanter. FORCES IN A TWO LEG STANCE • BW- equally distributed across both hips. • Lower limbs – 2/6th of BW • Upper limbs and trunk – 4/6th • Therefore each hip bares 1/3rd of BW • Direction of action of BW – directly perpendicular to ground. SINGLE LEG STANCE • Forces on head of femur = 2 to 3 times BW. • Rt LL supports BW + Weight of Left lower extremity , i.e. 4/6th + 1/6th = 5/6th • Rt Abductors exert downward counter balancing force. • Center of Gravity shifts to left. SINGLE LEG STANCE • Resultant force originates at an angle of 165 – 170 deg from vertical axis. • Resultant force = Force by abductors * Distance from GT to Centre of Head • This should be equal to partial BW * Distance from centre of head • If abductor lever arm is 5 cm • BW arm = 10 cm • Then, abd force * 5 cm = HATL * 10 cm SINGLE LEG STANCE • By calculation abd force comes to 2*BW = 140 kg • Joint load = HATL + abd force; 70+140=210 kg i.e. 3 times the BW. Bio Mechanics In Neck Deformities Coxa Vara • • • • Decreased neck shaft angle Increased abd lever arm Abd muscle length shortened Decreased joint forces across during one leg stance Coxa Valga • • • • Increased neck shaft angle Abd arm is shortened BW remains same Increased joint forces in hip during one leg stance • Valgus Osteotomy may move GT laterally improving the strength of contraction but joint forces may increase Biomechanics • Improving abductor function will decrease joint reactive forces Bio Mechanics of Walking Stick • Cane in Contralateral hand decreases JRF • Long moment arm makes so effective • 15% BW to cane reduces joint contact forces by 50% • Decreases the body turning movement and in turn R by exerting a force on a cane in the opposite direction. Bio Mechanics of CDH • GT is closer to the fulcrum • Force acts on the upper border of actabulum instead of being distributed over the whole • Excessive pressure exerted over a small area gives rise to degeneration Bio Mechanics of Trendlenberg Gait • Abd muscle force is in-adequate. Due to deformities that - Require increased force to maintain equilibrium - Or reduce the effectiveness of muscle contraction • By an adequate movement during single leg stance the centre of gravity can be shifted over the hip joint so that an unstable equilibrium exists and no abd muscle force is required. Bio Mechanics of OA • Contact area decreased due to deformity and loss of sphericity • Concentrates the compressive forces on to a smaller area • Patient decreases this load by tilting towards the affected side in stance phase thereby decreasing partial BW lever arm • Thus the rationale of decreasing BW in OA patients to decrease the abd force to counter balance and ultimately decreasing R Bio Mechanics of THR • Centralization of femoral head by deepening the acetabulum decreases the BW lever arm • Lateral attachment of GT lengthens the abd lever arm • Lengths of the lever arms are surgically changed to approach a ratio of 1:1 which reduces total hip load by 30% • Abd lever arm can be increased by increasing the medial offset of the femoral component or lateral or distal reattachment of GT. FEMORAL OFFSET?!! • The femoral offset is the distance from the centre of rotation of the hip joint to the line of action of the femur • The BW of the person will create a clockwise (negative) moment or torque about the centre of the hip. This torque must be balanced by an equal but opposite counterclockwise (positive) moment produced by the abductor muscle force. Offset (Cntd) • The moment arm of the abductor muscle is directly related to the magnitude of the femoral offset. As the offset increases in length the force required by the abductor muscles to balance the BW moment would reduce, thereby increasing the efficiency of the abductors. Bio Mechanics of THR (Cntd) • R is minimal if hip centre was placed in anatomical position • Adjustment of neck length is important as it has effect on medial and vertical offset.( (N) 25 – 50 mm) • Excessive medial offset results in stress fracture, loosening • Inadequate medial offset results in limp, impingement and dislocation Last but not the least!! “The importance of Bio Mechanics as the foundation for understanding the normal and pathalogical states of different joints and the treatment options involved cannot be under estimated.” REFERRENCES Campbell’s Operative Orthopaedic Surgery Textbook of orthopedics and traumaG.S.Kulkarni 4:2888 Mercer’s Orthopaedic Surgery 789-791 Current Orthopaedics (2006) 20, 23–31 Basic biomechanics of human joints: Hips, knees and the spine T.D. Stewarta,, R.M. Halla,b QUESTIONS???