Technical goals of TKR

1. Restore mechanical axis
2. Restore joint line
3. Balance ligaments
4. Restore Q angle.

Relevant knee anatomy & alignment

• Static alignment
  • The mechanical axis of the leg is formed by a line passing from the head of the femur to the centre of the ankle & passes through the centre of the knee joint.
  • The anatomical axis of the leg is 7 degrees valgus because of the offset created by the femoral neck.
  • The mechanical axis of the leg forms an angle of 3 degrees of varus compared with the midline vertical axis of the body.
  • The transverse axis of the knee joint is perpendicular to the midline vertical axis hence forms a 3 degree angle with the axis of the tibial shaft & a 10 degree angle with axis of the femoral shaft.
  • 65% of force across the knee joint occurs across the medial compartment.
• Dynamic alignment
  • A varus moment is imparted to the knee during normal gait that is resisted by the LCL, cruciates & ITB.

Contraindications to TKR

1. Active infection
2. Incompetent extensor mechanism
3. Compromised vascular status
4. Charcot joint (relative)

PCL retaining vs. substituting prostheses:

PCL substituting prostheses

• These use a tibial post that engages in a cam on the femur. 
• This prevents anterior femoral translation & with further flexion produces femoral roll back. 
• However, if the flexion gap is loose the femur can jump over the tibial post & dislocate. 
  • This generally needs to be reduced under anaesthesia.
• Theoretically:
  • The PCL retaining prosthesis should have a greater range of flexion because of femoral roll back but multiple studies have shown that the average flexion at long term review is similar. 
  • Femoral roll back
    • is posterior shift of the femoro-tibial contact point as the knee flexes. 
    • Rollback allows the femur to clear the tibia to provide further flexion.
    • For roll back to occur the tibial surface must be flat, but this leads to high contact stresses on the plastic. 
    • Increased congruency will lead to decreased contact stresses but will decrease the effect of roll back
  • The PCL retaining prosthesis should have a lower failure rate because of loosening because knees without PCLs need some mechanism to resist translation which must result in a higher stress being transferred to the bone cement interface but again at 10 years the results are similar
• PCL
  • is not normal in osteoarthritic patients, & fluoroscopic analysis shows that it doesn’t function normally.
  • It has been noted that patients with PCL retaining prostheses have a more symmetrical gait, especially during stair climbing.  Loss of the PCL may impair proprioception.
• Cruciate Retaining
  • PCL retaining prostheses don’t require as much femoral bone resection because they don’t have to accommodate a cam mechanism.
  • The joint line is maintained more easily in PCL retaining prostheses because retention of the PCL almost compels keeping the original joint line if the flexion & extension gaps are going to be balanced.
  • However, for all of these good things to happen the PCL must be properly tensioned; if too tight it will impede flexion & if too loose it will not be of any benefit.

The situations where one should strongly consider using a cruciate substituting prosthesis include:

1. PCL incompetence
2. Post patellectomy – the weakened extensors allow anterior femoral translation more easily
3. Inflammatory arthritis which may lead to late PCL rupture (however OKU 7 mentions study of RA patients with 97% 13 ysr).

Tourniquet

Tourniquet vs no tourniquet. No difference in:

1. Surgical time
2. Postoperative pain
3. Analgesic requirements
4. Drain output
5. Postoperative swelling
6. Incidence of wound complications
7. Incidence of DVTs

Exposure

• Options
  • Medial parapatellar
    • 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).
    • Tips
      • 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 & semimembranosis will allow greater external rotation & anterior translation of the tibial tubercle, which will in turn improve exposure & patellar eversion, & decrease the risk of patellar tendon avulsion.
  • Subvastus exposure
    • designed to avoid violating the extensor mechanism but provides inferior access to the lateral compartment.
  • Midvastus approach
    • compromise to these two approaches.

Difficult exposure

• Quadriceps turndown
  • It is a narrow inverted V incision based distally with the apex in the quadriceps tendon
  • essentially obsolete with no indications for its use. 
• Rectus snip
  • The incision in the quadriceps tendon is carried laterally & 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
  • a large portion of bone, from 3-6cm in length, should be used to promote reliable healing.
• Subperiosteal peel
  • A useful technique in the ankylosed knee
  • the entire soft tissue envelope is peeled from the bone & retracted posteriorly, allowing the distal femur to buttonhole forward

Femoral guides

• An intramedullary femoral jig should be used & the alignment confirmed with extramedullary methods if there is uncertainty e.g. unusual femoral bowing or wide intramedullary canal.
• Another advantage of using an intramedullary jig is that it avoids placing the femoral component in flexion or extension.
• A fluted intramedullary rod should be used because it reduces intramedullary pressure, & the entry hole should be overdrilled to 12mm.

Assessment of femoral valgus

• 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. 
• Very short patients may need an increased valgus cut angle, & very tall patients a decreased valgus cut angle.

Assessment of femoral rotation

• External rotation of the femoral component is necessary to optimize patellar tracking & to balance the flexion gap (without external rotation the lateral ligaments will be lax).
• Methods:
  1. Posterior condyles – often worn
  2. Epicondylar axis – can be difficult to define
     • The lateral epicondyle is the most prominent aspect of the lateral aspect of the distal femur & is just distal to a leash of condylar vessels
     • The medial epicondyle is located in the sulcus between the superficial medial collateral ligament & the deep collateral ligament
  3. Anteroposterior femoral axis (Whiteside’s line)
     • Whiteside says this is most reliable
     • Can be inaccurate in cases of femoral dysplasia or valgus knee
     • Use of Whiteside’s line & the epicondylar axis places the femoral component in the desired 3 degrees of external rotation.  If the posterior condyles are used, three degrees of external rotation will have to be added.
  4. Tibial shaft axis
  5. Ligament tension
• Tips
  • 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.

Anterior vs. posterior referencing of the femoral cut.

• Anterior referencing will lead to variations in the flexion gap;
• posterior referencing may lead to notching.

Tibial slope

• Increasing the posterior slope of the tibia will increase tendency towards anterior tibial translation but will help PCL deficit & increase the flexion gap.

Bone loss

• Contained defects may be filled with:
  1. Cement
  2. Cement & screws
  3. Graft
• Uncontained defects (e.g. medial tibial loss)

Balancing & releasing issues

Every effort should be made to balance the knee before increasing the constraint in a TKR. 

Varus release

1. Removal of osteophytes
2. Release of deep medial collateral ligament
3. Release of posteromedial corner with semimembranosis
4. Superficial MCL
5. PCL (rarely)
6. Popliteus (rarely)

Valgus release

1. Removal of osteophytes
2. Extension tightness – extracapsular release of ITB
3. Flexion tightness – release of posterolateral structures including popliteus from femur
4. PCL

Posterior release

• Get rid of all osteophytes from behind the femoral condyles. 
• Use Bristow to release from behind capsule.
• If this isn’t enough, may need to take more off distal femur.
• When extreme deformity cannot be balanced with controlled ligament release the options are:
  1. Correct & balance to the maximum degree & then brace the knee for about 6 weeks postoperatively. 
     • This is an option only in fixed varus knees
  2. Reconstruct the elongated ligament
  3. Use a prosthetic device such as a constrained condylar knee that provides for collateral ligament substitution
  4. If the femur is being pushed backwards i.e. the tibia is pulled forwards ensure the PCL isn’t too tight & if so release it.
  5. Taking more off the posterior aspect of the femur & increasing the posterior slope of the tibia will increase the flexion gap & effectively increase flexion.

Sagittal plane balancing

• The goal of sagittal plane balancing is to obtain equal flexion & 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.

Scenarios/problems/solutions

1. Tight in extension & tight in flexion. 
   • Implies not enough tibia removed.  Take more tibia
2. Tight in extension & OK in flexion. 
   • Implies not enough distal femur removed. 
   • Release posterior capsule & take more distal femur. 
   • Note that if too much distal femur is removed the knee may become loose in flexion, as the collateral ligaments are relatively too long. 
   • The patient may require a constrained condylar prosthesis in this situation.
3. OK in extension but tight in flexion. 
   • Implies not enough posterior femur removed. 
   • Need to take more off posterior femur i.e. downsize. 
   • Can also increase posterior slope of tibia, & release the PCL.  If the anterior tibia lifts off in knee flexion the PCL is too tight.
4. Loose in extension but OK in flexion. 
   • Implies too much distal femur removed. 
   • Need to augment distal femur
5. OK in extension but loose in flexion.
   • Implies too much posterior femur removed. 
   • Need to increase femoral size & use augments or cement in the posterior gap.

Patella issues

Tracking

1. The patella dome should be medialized as much as possible, & be in the proximal part of the patella.
2. Internal rotation of the tibial tray will externally rotate the tibial tubercle & should be avoided.
3. Internal rotation or medial translation of the femur will move the trochlea medially & should be avoided.

• Note: if the patella appears to be maltracking is worthwhile releasing the TQ & reassessing the tracking, as around 50% of knees will show an improvement in tracking with the release.
• If a lateral release is required should try to preserve the lateral superior geniculate artery, which is located at the musculotendinous junction of vastus lateralis.

Patella baja

• This is most commonly seen after HTO.
• Techniques to manage this include:
  1. Place dome high up on patella then trim off distal patella
  2. Lower joint line by taking more tibia off & augmenting distal femur
  3. Cut off impinging tibial plastic
  4. Proximal displacement osteotomy of tibial tubercle.

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 & audible clunk. 
• Treatment
  • open or arthroscopic debridement.

Decision to resurface patella

• This is a controversial area. 
• Indications
  • The patella should probably be resurfaced in patients with
    • inflammatory arthritis,
    • Paget’s disease,
    • crystal arthropathy.

Wound closure

• Wound closure in 90 degrees of flexion resulted in significantly better flexion at one year followup when compared with closure in extension (Emerson)

CPM

• May be associated with an early improvement in ROM, but there is no significant difference at 1 year, & there is no difference in the rate of MUAs.
• CPM is associated with increased anaesthetic requirements & increased wound drainage.
• If more than 40 degrees, decreases oxygen tension in the wound.