- Assistance provided by Michael Berend MD.
- Discussion:
-
flexion and extension gaps:
- this is determined preoperatively by the height of the tibial surface and height of the posterior femoral condyle;
- the flexion gap often ends up being larger than the extension gap, and this needs to be determined before the primary components are removed;
-
medullary reaming:
- note that most revision hip systems base their femoral cuts off of the previously inserted straight medullary reamer;
- if the reaming hole is inserted too far anteriorly, then their will be an excessively large
flexion gap, and the anterior flange of the femoral component
will lie proud off of the femoral surface;
- likewise if the reaming hole is placed too far posteriorly, then the flexion gap will be narrowed;
- consider measuring the distance between the stem of the femoral component and the inner edge of the anterior flange;
- this distance can then be used to help guide the proper position of the reaming hole (ie, by translating the reaming hole the proper
distance from the anterior femoral surface);
- after reaming to 12 mm, continue to procede slowly by 1 mm increments;
- reaming should cease once firm resistance is encountered;
- it is not necessary to have direct cortical contact, inorder to avoid excessive bone loss;
- generally, stem diameter will be the same size as the reaming diameter;
-
depth of reaming:
- if straight stems are used, do not ream past 190 mm inorder to avoid impinging the femoral anterior bow;
- if a longer stem is required, then it should be curved;
- bone will often be deficient distally and posteriorally, so be prepared to augment these areas;
- insert IM rod with appropriate sleeve and place IM cutting guide (we prefer 5 degrees of valgus);
- remove 1-2mm of bone distally
- perform anterior and posterior cuts;
- both the anterior and posterior cuts should be parallel to the epicondylar axis;
- be aware that increased external rotation will improve patellar tracking, increase the medial flexion gap and decrease the lateral flexion gap;
-
insertion of the trial femoral component:
-
rotation of the femoral component:
- if their has been optimal patellar tracking, then the surgeon should accept the orientation of the pre-existing femoral bone cuts;
- if patellar tracking is not acceptable, then the surgeon will have to change the orientation of the anterior, posterior, and chamfer cuts (ie, more external rotation is required);
- use the femoral epicondyles to judge neutral rotation of the femoral component;
-
flexion and extension gaps: final adjustment;
- as stated, the femoral medullary stem will have the most significant effect on flexion and extension gaps;
- if the
flexion gap is greater than the extension gap, then there will be no easy solution;
- if the
extension gap is greater than the flexion gap, then the femoral trial can be augmented w/ wedge spacers, but just ensure that the patient can achieve full extension;
-
evaluate and manage bone defects:
- probably in most cases 4-6 mm distal femoral augments are required, especially when the extension gap is greater that 16-18 mm;
- consider inserting trial components and noting when the patellar component impinges on the tibial component;
- if patellar-tibial impingement at knee flexion angles less than 110 deg probably indicates
patella baja or raising of the joint line;
- in this situation additional distal femoral augments are required;
- be prepared, to add wedges to the posterior condyles so that no gaps remain between posterior femoral components and the bone surface;
- use a caliper inorder to meansure bone gaps, and re-trial with an augmented femoral component;
