What is Arthroplasty:
Arthroplasty (literally “surgical repair of joint”) is an operative procedure of orthopaedic surgery in which the arthritic or dysfunctional joint surface is replaced with something better or by remodeling or realigning the joint by osteotomy or some other procedure.
Arthoplasty Introduction:
Total joint replacement
Increasing demands to understand biomechanics of joint
To improve normal joint function
Lower probability of implant failure
Reduce load on total joint replacement during daily activites
Design implant that can withstand loads.
Younger population
What is Arthoplasty Mechanical problem associated with total joint replacement:
Mechanical problem associated with total joint replacement
Wear of bearing surfaces
Mechanical failure of implant
Loosening of implant from bone
Dislocation of implant at articulating surfaces
What is Arthoplasty Goal of joint replacement:
Long term restoration of function and
pain relief .Several mechanical challenges to achieve this goals
To meet one design objective, compromise on another design objective
For greater rotatory stability…..medulary canal fill implant ,heavier and stiffer implants
Stress shielding.
Forces acting at hip and knee
Forces acting at hip and knee are dependent upon
External forces acting on the joints from outside environment
Internal forces generated by muscle contraction
Measurement of joint forces:
Implant transducer
Inverse dynamics( biomechanical analysis is to know what the muscles are doing: the timing of their contractions, the amount of force generated (or moment of force about a joint), and the power of the contraction – whether it is concentric or eccentric
Analytical method
Solving muscle and contact force at hip and knee remain Indeterminate problems
Solution of indeterminate problem:
Ist Reduction method=group muscles into functional units
2nd Optimization method=force distribution among muscles in a manner that
minimum stress
Maximum muscle endurance
Maximum joint reaction force are optimized
Forces at the Hip Joint:
The peak resultant forces during gait measured with strain gauged prosthesis have ranged from 1.8 to 4.36 times body weight
These forces increase with walking speed
2 peaks=one during early stance and 2nd in late stance phase
Failure not only due magnitude of force but also due to cyclic nature of the load
Rotational Moments about the implants:
Out-of-plane loads may be detrimental to both initial as well as long term implant stability especially uncemented stems
Excessive bone-implant motion=prevention of bone in-growths in porous coating
Failure of biological fixation.
Large torsional movements measured in vivo during daily activities reach average experimental strength of implant determine by invitro testing
In vitro testing of prosthesis implanted in femure indicate that amount of initial bone implant motion sensitive to off axis loading that occur during stair climbing and rising from chair.
Walking with decreased ROM during daily activities may minimize out of plane motions
Decreased sagital plan motion in patient of THR may benifical for implant stability by reducing rotational movement about implant stem
Decreased ROM as adaptive response to decrease torsional micromotion produced by out of plane loads ,when hip reaches greater range of flexion
Reconstructed joint geometry:
Alteration in joint anatomy impact on hip biomechanics by altering: The contact area
The contact force
And the strength and moment-generating capacity of the muscles
Mechanical ability of abductors affected by head –neck angle ,neck length and joint centered position
All altered during THR. A decreased head-neck angle (varus hip) increases the mechanical advantage of the abductors.Joint contact forces should be minimized with small angle
Decreased head-neck angles also improve join stability through increase congruence by turning the femoral head into the acetabulum
Moving the greater trochenter laterally also increases the mechanical advantage of the abductors
Clinically increased abductor/adductor strength has been associated with increased neck length and a more distal greater trochenter position
Forces at knee joint:
Knee has to depend on surrounding tissue for stability
The peak resultant forces during gait have ranged from 3 to 7 times body weight
The magnitude and the cyclic nature of the compressive force in the tibiofemoral are important consideration in the design of the total knee replacement
Failure as a result of implant and interface interaction and cyclic fatigue.
The portion of the tibial plateau that is loaded varies with knee flexion angle.
Smaller the contact area-larger will be the peak stress
Tractive rolling of femur on tibia
Different gait pattern-different tractive forces
Medial-Lateral load distribution:
Tibial component loosening
Load imbalance between medial and lateral tibial surface
Early designs were not sufficient to sustain this load difference
During walking approx 70% of the load across knee joint is normally sustained by the medial compartment of th knee
Adduction moment
Varus alignment are more likely to have a substantial load imbalance that creates stresses that could eventually lead to tibial component loosening
Increased wear has been demonstrated in the medial compartment of the knees in varus preoperatively and in the lateral compartment for those in valgus preoperatively
To address these problems:
Metal backing of the tibial implant/polyethylene surface
Modification of surgical approach
Proper alignment
Patellofemoral joint and loads:
The magnitude of retro patellar forces as well as the contact area on the retro patellar surface varies with knee flexion angle
Replication of normal patellofemoral anatomy-essential
Elevated joint lines, which effect patella function and patella subluxation, have been correlated with wear patterns.
