Technical goals of TKR
Contraindications to TKR
Mechanical axis of the lower extremity passes through centre of hip, knee, and ankle.
Mechanical axis of femur and tibia lie on this line.
Anatomical axis lies 5° varus to the mechanical axis.
Thus to cut perpendicular to the mechanical axis of the leg, you must make the distal femoral cut in 5° of valgus (referenced from an IM rod).
This will correspond to a cut 3° varus to the articular surface.
Anatomical axis is parallel to the mechanical axis, which corresponds to the mechanical axis of the lower limb (and the intramedullar canal of the tibia).
Thus referenced from an IM rod, the tibial cut is perpendicular.
This corresponds to a cut 3° valgus to the articular surface.
This is collinear with the mechanical axis of the lower limb.
This is perpendicular to the epicondylar axis.
65% of force across the knee joint occurs across the medial compartment.
Bone cut referencing
Various landmarks are used to make femoral and tibial bone cuts which result in joint surface articulating perpendicular to the mechanical axis of the lower extremity in both flexion and extension.
Distal cut is referenced as above from an intramedullary guide rod.
An intramedullary femoral jig should be used and the alignment confirmed with extramedullary methods if there is uncertainty (for example; unusual femoral bowing or wide intramedullary canal).
A fluted intramedullary rod should be used because it reduces intramedullary pressure, and the entry hole should be overdrilled to 12mm.
The distal cut is 5-7° valgus relative to this intramedullary (anatomical) axis.
7 degrees of valgus in varus knees
5 degrees of valgus in valgus knees
6 degrees of valgus in non-deformed knees.
Obese patients get a 5 degree cut to avoid rubbing their knees together.
Shorter patients may need an increased valgus cut angle, and taller patients a decreased valgus cut angle.
There are 3 possible landmarks used as references for the posterior cut:
Provide excellent referencing provided they are not altered by the disease process.
Cut is made in 3° of ER relative to normal posterior condyles.
They are particularly unreliable in valgus knees (where the wear is primarily from the LFC).
Lies perpendicular to the mechanical axis (thus any cut must be parallel to it).
The lateral epicondyle is the most prominent of the lateral aspect of the distal femur and is just distal to a leash of condylar vessels.
The medial epicondyle is located in the sulcus between the superficial medial collateral ligament and the deep collateral ligament.
Unfortunately it is anatomically inconsistent and hard to determine.
It is used as a reference and is reserved mainly for revision TKR where there is excessive bone loss.
A-P axis (Whiteside’s line)
Defined by a line drawn from the deepest part of the trochlea groove anteriorly to the centre of the intercondylar notch (which in turn is consistent with the lateral fibres of the PCL insertion).
This is an excellent reference corresponding to the mechanical axis, thus the posterior cut needs to be perpendicular to this line.
Whiteside says this is most reliable.
It can be inaccurate in cases of femoral dysplasia or valgus knee.
Use of Whiteside’s line and the epicondylar axis places the femoral component in the desired three degrees of external rotation. If the posterior condyles are used, three degrees of external rotation will have to be added.
In a typical medial OA knee with correct external rotation of the femoral component more medial posterior femoral condyle than lateral condyle will be removed. In a valgus knee posterior erosion of the lateral femoral component may lead to unwitting internal rotation of the femoral component.
Intraemdullary or extramedullar jigging may be used.
There is no proven benefit of either (similar results).
Extramedulary needed if there is significant tibial bowing (present in 2/3 of valgus knees).
Cut is perpendicular.
Corresponds to 3° valgus to the articular surface.
Cut is parallel to the fibula in the sagittal plane.
In coronal plane jig points to:
Centre of ankle.
Which is more medial than the mid-malleolar distance.
Line of 2nd ray.
Anterior vs. posterior referencing of the femoral cut.
Anterior referencing will lead to variations in the flexion gap.
Posterior referencing may lead to notching.
Femoral bone plug
A femoral bone plug decreases blood loss by 20-25%.
Increasing the posterior slope of the tibia will:
Insall feels that slotted capture guides reduce human error, although they make it difficult to see the tip of the saw blade.
Milling frames create the smoothest bone surface and also generates less heat, resulting in less damage to the bony cut surface.
May be filled with Cement, Cement and screws or Graft.
For example, medial tibial loss.
Can be reconstituted with the technique of Windsor.
Convert the medial tibial bone loss defect to a flat surface with a saw cut, and then fix a bone off. Cut to the cut surface with threaded K wires or screws.
This construct should be protected with a stem.
A varus moment is imparted to the knee during normal gait that is resisted by the LCL, cruciates and ITB.
The routine approach is a medial parapatellar approach. This should extend distally to 1cm medial to the tibial tubercle (to avoid a scar directly over the tubercle).
Division of the lateral patellofemoral ligament will aid in patellar eversion.
Running a Bristow posteriorly on the medial aspect of the tibia reflecting the medial capsule, deep MCL and semimembranosis will allow greater external rotation and anterior translation of the tibial tubercle, which will in turn improve exposure and patellar eversion, and decrease the risk of patellar tendon avulsion.
The subvastus exposure is designed to avoid violating the extensor mechanism but provides inferior access to the lateral compartment.
A midvastus approach is a compromise to these two approaches.
Insall says that this is essentially obsolete with no indications for its use.
It is a narrow inverted V incision based distally with the apex in the quadriceps tendon.
The incision in the quadriceps tendon is carried laterally and proximally into the vastus lateralis.
The incision can be combined with a lateral retinacular release, with the blood supply to the patella entering via the superior lateral genicular vessels.
Tibial tubercle osteotomy.
Favoured by Whiteside; he emphasises that a large portion of bone, from 3-6cm in length, should be used to promote reliable healing.
A useful technique in the ankylosed knee; the entire soft tissue envelope is peeled from the bone and retracted posteriorly, allowing the distal femur to buttonhole forward.
OKU 7 mentions a well matched prospective study where there was no difference in:
Serves multiple functions through the flexion-extension arc:
Composed of 2 bands:
Tight in flexion.
Tight in extension.
It is often tight in the varus knee and stretched in the valgus knee.
When tight the XS femoral rollback is in flexion.
It can be partially released.
This may result in loss of function in A-P stability, whilst maintaining its secondary functions of rotational and varus-valgus stabilisation.
Release with a bony sleeve from the tibia using an osteotome.
If completely defunctioned a more conforming tibial insert can be used to compensate for its action of A-P stability.
PCL retaining vs. substituting prostheses:
Theoretical benefit of retaining PCL – In Practice:
PCL greater range of flexion due to rollback
Effective rollback requires a flat surface which in turn high contact stresses. Thus most poly inserts are curved to improve congruency but also limit rollback
PCL retention lower failure rate due to less stress being transferred to bone cement by A-P translation
10 year survival rates are similar in PCL retaining and sacrificing prostheses.
PCL aids proprioception.
Patients do have a more symmetrical gait, particularly climbing stairs.
PCL retention aids maintainance of the normal joint line.
Less femoral bone resection.
In prostheses substituting the PCL with a cam mechanism, more femoral bone must be resected.
Indications for PCL substitution
The situations where one should strongly consider using a cruciate substituting prosthesis include:
Post patellectomy – the weakened extensors allow anterior femoral translation more easily.
Inflammatory arthritis which may lead to late PCL rupture (however OKU 7 mentions study of RA patients with 97% 13yrs).
Mechanisms for PCL substitution
Tibial post/femoral cam.
These use a tibial post that engages in a cam on the femur.
This prevents anterior femoral translation and with further flexion produces femoral roll back. However, if the flexion gap is loose the femur can jump over the tibial post and dislocate. This generally needs to be reduced under anesthesia.
Highly conforming poly insert, for example, the ‘conforming plus’ poly in the Profix system.
Balancing and releasing issues
Every effort should be made to balance the knee before increasing the constraint in a TKR.
Assessment is made in flexion and extension, with different releases required depending on the specific imbalance present – see ‘Varus knee’ and ‘Valgus knee.’
Release specific structures based on their function through the flexion arc.
Tight in Flexion Tight in extension
Anterior 1/2 superficial MCL Post 1/2 superficial MCL
Medial PCL (secondary stabiliser) Postero-medial capsule
Lateral Popliteus Popliteus
Biceps, gastro, PLC
When extreme deformity cannot be balanced with controlled ligament release the options are:
Both Guitronich and Cumberland leave their knees fairly loose, particularly in the elderly.
Sagittal plane balancing
The goal of sagittal plane balancing is to obtain equal flexion and extension gaps.
If the gaps are symmetric, then balancing problems lie in the tibia. If they are asymmetric the balancing problems lie in the femur.
The patella dome should be medialised as much as possible, and be in the proximal part of the patella.
Internal rotation of the tibial tray will externally rotate the tibial tubercle and should be avoided.
Internal rotation or medial translation of the femur will move the trochlea medially and should be avoided.
Note: if the patella appears to be maltracking it is worthwhile releasing the TQ and reassessing the tracking, as around 50% of knees will show an improvement in tracking with the release.
If a lateral release is required, try to preserve the lateral superior geniculate artery, which is located at the musculotendinous junction of vastus lateralis.
This is most commonly seen after HTO.
Techniques to manage this include:
Patella clunk syndrome
This is seen in patients with PCL substituting designs. A fibrous lesion can develop at the proximal pole of the patella which catches in the box of the femur as the knee extends from around 40 degrees of flexion. It then pops out with a palpable and audible clunk. Treatment is with open or arthroscopic debridement.
This is a controversial area.
Absolute indications for resurfacing
Relative indications for resurfacing
RCT in JBJS 2002 Wood et al using MG II prosthesis.
This is a significantly higher incidence of anterior knee pain if patella not resurfaced.
31% knee pain without resurfacing, 16% with resurfacing.
Weight, not BMI was only variable associated with development of anterior knee pain.
RCT in JBJS 2003 by Waters et al using Press Fit Condylar prosthesis at 5 years found 25% rate of anterior knee pain in patients without resurfacing and 5% rate in patients with resurfacing.
The thickness of the patella must be maintained if it is resurfaced.
Minimum acceptable thickness is 10mm.
Wound closure in 90 degrees of flexion resulted in significantly better flexion at one year followup when compared with closure in extension (Emerson).
May be associated with an early improvement in ROM, but there is no significant difference at 1 year, and there is no difference in the rate of MUAs.
CPM is associated with increased anesthetic requirements and increased wound drainage.
If more than 40 degrees, it decreases oxygen tension in the wound.