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Joints, Arthroplasty Introduction its biomechanics

Arthroplasty:

Arthroplasty (literally “surgical repair of joint”) is an operative procedure of orthopedic 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.

Introduction:

Total joint replacement

Increasing demands to understand the biomechanics of joint

To improve normal joint function

Lower probability of implant failure

Reduce load on a total joint replacement during daily activities

Design implant that can withstand loads.

Younger population

A mechanical problem associated with total joint replacement:

The mechanical problem associated with total joint replacement
Wear of bearing surfaces
Mechanical failure of an implant
Loosening of an implant from the bone
Dislocation of an implant at articulating surfaces

The goal of joint replacement:

Long-term restoration of function and
pain relief.Several mechanical challenges to achieve this goal
To meet one design objective, compromise on another design objective
For greater rotatory stability…..medullary canal fill implant, heavier and stiffer implants
Stress shielding.

joints

Forces acting at hip and knee

Forces acting at hip and knee are dependent upon
External forces acting on the joints from an 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

The solution to an 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
The maximum joint reaction force is 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 a due magnitude of force but also due to the 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 the porous coating
Failure of biological fixation.

Large torsional movements measured in vivo during daily activities reach average experimental strength of implant determine by in-vitro testing
In vitro testing of the prosthesis implanted in femur indicate that amount of initial bone implant motion sensitive to off-axis loading that occurs during stair climbing and rising from the chair.

Walking with decreased ROM during daily activities may minimize out of plane motions
Decreased capital plan motion in a patient of THR may beneficial for implant stability by reducing rotational movement about implant stem
Decreased ROM as an adaptive response to decrease torsional micromotion produced by out of plane loads, when hip reaches a greater range of flexion

Reconstructed joint geometry:

Alteration in joint anatomy impact on hip biomechanics by altering. The contact force and the strength and moment-generating capacity of the muscles

Mechanical ability of abductors affected by the 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 a small angle

Decreased head-neck angles also improve join stability through increase congruence by turning the femoral head into the acetabulum
Moving the greater trochanter 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 trochanter position

joints

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 an 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 the femur on the 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 the kneejoint

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.