- Theoretical Considerations:
- distance from posterior reesction plane to anterior resection plane must equal the internal dimension of
femoral component being used;
- this cut must be made at a distance from the anterior cut equal to inner anterior-posterior dimension of femoral component used;
- depending upon inner geometry of implant being used, anterior and posterior resection lines should
either be parallel or diverge slightly;
-
flexion gap:
- should be rectangular in shape rather than trapezoidal, in order to achieve this, femoral template is externally rotated
on distal femur until it become parallel with the cut tibia surface;
- this results in more bone resection from the posterior medial femoral condyle than the posterior lateral femoral condyle;
- amount of posterior femoral condylar bone removed should equal thickness of posterior condylar dismension of implant being used;
-
rotatory alignment of cutting guide:
- rotatory alignament of the AP cutting jig is normally based upon the position of the posterior femoral condyle;
- in some of the older TKR designs, it was recommended that the tibia be cut in slight varus (to reproduce the anatomy);
- w/ this type of design, it is necessary to remove an equal amount of bone should be removed from each posterior femoral condyle;
- this type of design does not allow for external rotation of the femoral component (since this would created a trapezoidal
shaped flexion gap), and this has a negative effect on patellar tracting;
- newer TKR designs recommend that the proximal tibia be cut at 90 deg (as opposed to 3 deg of varus), and therefore it is necessary
to make the AP cuts in 3 deg of external rotation inorder to keep a rectangular flexion gap;
- this slight external rotation has a beneficial effect on patellar rotation;
- when the tibia has been resected at 90 deg, however, slightly more bone must be removed from the medial than from lateral condyle (that
is guide must be slightly externally rotated) in order to reestablish rectangularity of
flexion space;
- femoral template should not be internally rotated, since this moves patellofemoral groove of
femoral component medially,
making it more difficult for laterally placed patella to capture groove;
- malrotation of the
femoral component may lead to patellar dislocation or subluxation;
- Technique:
- proper size of the AP and Chamfer cuts is confirmed w/ the
AP sizing guide;
-
rotational alignment of the cutting guide:
- when the posterior femoral condyles have been eroded, ensure that the cutting device is perpendicular to the femoral trochlea;
- always ensure that the anterior cut will not notch the femoral cortex;
- cutting block is placed & anterior, posterior, & chamfer cuts performed w/ retractors placed to protect
collateral ligaments, and the patellar tendon;
- bone is removed & cuts rechecked and noted to be intact;
-
pitfalls:
- inaccurate sagittal orientation that places femoral component in too much extension may notch
the anterior femur and can lead to later supracondylar fracture of the femur;
-
femoral component placed in too much flexion alters the kinematics of the knee and may decrease extension;
- Resection of the Anterior Femur:
- size of the component must be chosen so that the anterior resection does not notch femoral cortex;
- such inadvertent notching can set up a stress riser in femur w/ the potential of later frx;
- this cut intersects the femoral cortex at the point where the trochlear surface merges into the shaft;
- any resection posterior to this point will notch anterior femur;
- consequences of notching the femur:
- in the study by ML Lesh MD et al (JBJS-A 82-A: 1096-101, 2000), the authors performed a biomechanical study to determine
the consequences of notching the femur;
- authors noted that femoral notching significantly lessens the load to failure following TKR and influences the subsequent fracture pattern;
- they recommended great care in the postop PT for these patients and recommended that these patients should not be subjected to knee manipulation;
- references:
-
The Effect of Femoral Notching During Total Knee Arthroplasty on the Prevalence of Postoperative Femoral Fractures and on Clinical Outcome.
- Resection of the Posterior Femur:
- the medial condyle extends below the epicondylar axis further than the lateral condyle and therefore,
more of the medial condyle will be resected;
- if a PCL retaining component is to be used, consider using a narrow blade saw so that there will
be less chance of injuring the PCL;
- maximal flexion of the knee will help protect the posterior N/V bundle;
- after the sawing is complete, small remnant portions of uncut bone may hold the posterior fragment to the femur;
- pass a 1 inch straight osteotome into the saw cut and gently tap it until the posterior fragment is freed;
- Chamfer Cuts:
- Trial Femoral Components:
- trial femoral component are placed at this point;
- if the trial does not sit down perfectly, then the cuts need to be repeated;
- take the knee thru flexion and extension and note the propensity for patellar subluxation;
- if there is a tendency for lateral subluxation, then the trial should be slightly lateralized;
- even slight medialization of the femoral component can cause tightening of the lateral retinaculum leading to patellar maltracking;
- even if a
posterior stabilized prosthesis is going to be used, insert the PCL retaining trial component at this step, since
it is more effecient to recut the distal femur at this point than to have to do it once the box cuts have been completed;
- Box Cuts: (for
Posterior Stabilized Componenets)
Effect of a cooled saw blade on prosthesis fixation. Randomized radiostereometry of 33 knee cases.
S Toksvig-Larsen and A Lindstrand.
Acta Orthop. Scandinavica. Vol 65: 1994. p 533-537.
On the problem of heat generation in bone cutting. Studies on the effects on liquid cooling.
S Toksvig-Larsen.
JBJS. Vol 73-B.(1) 1991. p 13-15.
The effect of prosthetic patellar thickness and anterior femoral surface on limiting flexion in total knee arthroplasty.
J Holtgrew et al.
Trans. Orthop. Res. Socl. Vol 14. 1989. p 369.
Why Posterior Cruciate-Retaining and Substituting Total Knee Replacements Have Similar Ranges of Motion
THE IMPORTANCE OF POSTERIOR CONDYLAR OFFSET AND CLEANOUT OF POSTERIOR CONDYLAR SPACE.
- Implant Selection:
Osteonics
- Johnson and Johnson:
- Box Cuts: (for
Posterior Stabilized Componenets)
