- See:
-
Joint Line Position
-
Patella Infera:
- Discussion:
- elevation of the joint line occurrs when only very small amount of tibial plateua surface is removed and "space" for the componenets
created by removing portion of the distal femur thicker than the
femoral component;
-
extension gap is therefore larger than the
flexion gap;
- as the knee flexes beyond 45 to 60 deg, the collateral ligaments become increasingly tight;
- they also become extremely tight if flexion beyond 90 deg is attempted;
- best way to properly position the joint line is to begin by removing amount of distal femur equal in thickness to thickness of implant;
-
quantification of joint line malposition:
- if the opposite knee has not undergone TKR then measure the distance from the adductor tubercle to the distal edge of the femoral component;
- if the opposite knee has undergone previous TKR (or has deformity) then the optimal joint line position often lies two finger breadths above the tibial tubercle;
- Consequences of Joint Line Elevation:
-
decrease in knee flexion:
- w/ a PCL retaining prosthesis elevation of joint line from 1-6 mm will significantly reduce knee flexion;
- occurs due to tightness of
PCL and
posterior capsular structures because of distal tibial displacement with knee flexion;
- elevation of joint line of 2, 4, and 6 mm results in mean flexion limits of 105 deg, 93.3 deg, and 74.1 deg, respectively;
-
posterior stabilized prosthesis is more tolerant of joint line elevation;
- sectioning of
PCL w/ joint line elevated 6 mm resulted in mean flexion of 104.6 deg;
-
effects on the patella:
- where as
posterior stabilized prosthesis is more tolerant of changes in the joint line position in terms of knee flexion, patella tracking remains a problem;
- joint line elevation results in iatrogenic
patella baja w/ inefficient quadricep mechanism;
- patella may impinge on the anterior edge of the tibial polyethylene component (esp w/ posterior stabilized components);
- eversion of the extensor mechanism during revision surgery may be more challenging with true patella baja compared with pseudo-patella baja;
- in the report by RP Grelsamer et al, the author makes the point that this situation is really a pseudo-patella baja because the patella is in
its normal position relative to the femur;
- the author makes the point not to attempt to detect true patella baja with either the Blackburne-Peel or the Caton-Deschamps ratio in the setting of a
prosthetic knee because these ratios are affected adversely by the position of the joint line;
- ref: Patella baja after total knee arthroplasty: Is it really patella baja?
Ronald P. Grelsamer, January 2002 Volume 17 Number 1
- Clinical Application:
- its important in revision situations, that the
femoral component be seated distally enough to come close to restoring the original level of knee joint line;
- use the adductor tubercle as a radiographic landmark as compared to the opposite knee, inorder to help judge the amount of elevation;
- alternatively use the distance from the tip of the fibular head to the joint line;
- significant elevations may require
bone graft or augmented femoral components;
The influence of patellofemoral location on function of the knee in patients with the posterior stabilized condylar knee prosthesis.
HE Figgie et al.
JBJS Vol 68-A 1986. p 1035-1040.
Joint line displacement of a total knee replacement results in reduction of knee flexion.
JF Cummings et al.
Trans. Orthop. Res. Soc.
Vol 15. 1990. p 280.
