Keywords

Overview

  • TKA (total knee arthroplasty): For management of knee arthritis when conservative measures (NSAIDs, physical therapy, injections of corticosteroid ± viscosupplementation, and off-loader bracing) have failed. TKA is primarily a pain-relieving operation.

TKA Components

  • Most TKA consists of femoral, tibial, and polyethylene components (tibial/patella).

TKA Design Concepts

Designs are generally categorized by level of constraint. Constraint refers to the ability of a prosthesis to stabilize the knee substituting for loss of ligament stability or bone. This can be in anterior/posterior, flexion/extension, or varus/valgus planes. Increasing constraint of components grants greater stability at the cost of longevity of the implant. Highly constrained implants put higher forces across the components leading to faster wear and failure. There are four major categories of constraint.

Cruciate Retaining (CR) Knee

  • Least constrained design. Resects ACL and retains native PCL. Depends on native ligaments to provide stability. No box cut in the femur. Theoretically increased longevity due to low constraint, improved proprioception with native PCL and tissue, and risk of PCL rupture and instability. If PCL rupture occurs, it needs to revise femoral component to make box cut to accommodate polyethylene post for posterior-stabilized (PS) design.

Posterior-Stabilized (PS) Knee

  • Resects ACL/PCL and cuts femoral cam/box to accommodate tibial polyethylene post during flexion. Provides partial constraint anterior to posterior (AP) substituting for PCL. Resection of PCL provides easier exposure and is easier to balance compared to CR knees. Unique complications associated with PS design:

    • Cam jump “jumping the post”: If PS knee is loose in flexion or can hyperextend, then polyethylene post can escape the femoral box and dislocate. Requires initial reduction and eventual revision to provide appropriate flexion/extension balancing to prevent further dislocation.

    • Patellar clunk: Fibrous tissue (nodule) superior to the patella (on quad tendon undersurface) gets caught in femoral box in the trochlea. Creates catching or popping sensation when extending the knee from a flexed position. Usually engages at roughly 40°. Risk factors are a small patellar component or valgus knee. Treat with open or arthroscopic resection of fibrous nodule.

    • Polyethylene post wear: Tibial post provides an additional wear surface.

Constrained Non-hinged Knee

  • A deeper femoral box and larger polyethylene post provide even greater constraint/stability. Provides partial (AP), varus/valgus, and some rotational stability. Used for knees with LCL or MCL deficiency, bone loss, or flexion gap laxity. Increased constraint means more rapid wear and loosening. Used in revisions/more severe tissue loss/deformity.

Constrained Hinged Knee

  • Highest level of constraint has linked femoral and tibial components (hinge). Used for most global ligamentous laxity, severe deformity, revisions, or oncologic procedures with significant bone loss. High level of constraint significantly reduces longevity of implant.

Approaches for TKA

Medial parapatellar approach through midline incision in the skin is most common approach for TKA. If multiple incisions from prior surgery, use the most lateral as medial blood supply to tissue flaps.

Minimally Invasive Approaches

  • Midvastus/subvastus approaches attempt to preserve extensor mechanism. Both are more limited exposure and should be avoided in obese patients or revisions.

Extensile Approaches/Exposure

  • Quadriceps snip: Oblique 45° angle cut into the vastus lateralis from proximal end of medial parapatellar approach. Increases exposure and no long-term difference in rehabilitation or postoperative protocols/weight bearing/activity

  • Tibial tubercle osteotomy: Allows greater exposure than quadriceps snip. Does not disrupt the quad like in a V-Y turndown but does have risk of tibial tubercle avulsion after repair and nonunion of osteotomy and often requires changing post-op protocol for weight bearing/rehabilitation

  • V-Y turndown: Normal medial parapatellar incision that proximally extends obliquely down the vastus lateralis tendon from proximal to distal. Provides excellent exposure and preserves patellar tendon/tibial tubercle but disrupts post-op rehab and has a risk of extensor lag and disruption of the quad mechanism

TKA Alignment

  • Normal anatomy: Distal femur 5–7° of valgus, proximal tibia 2–3° of varus. Try to establish mechanical alignment of zero.

    • Mechanical axis femur: Line connecting center femoral head to the intercondylar notch

    • Mechanical axis tibia: Center of the proximal tibia to center of the ankle.

    • Anatomic axis femur/tibia: Line bisects the medullary canal of the femur/tibia.

    • Q angle: Angle between ASIS to center of the patella and the axis of the patellar tendon to tibial tuberosity. Increased Q angle produces lateral force on the patella/maltracking. If severe enough, it can cause pain/dislocation. Patellar maltracking is one of most common complications of TKA.

      • NEVER internally rotate femoral or tibial components. Increases Q angle!

      • DO NOT medialize femoral component or place patellar component lateral.

TKA Balancing (Need to Balance Coronal and Sagittal Planes)

Coronal Balancing

  • Varus deformity most common in TKA. Typically tight medial side with lax lateral side. To correct resect osteophytes, meniscus, deep MCL, and capsular attachments. If still unbalanced, then release posteromedial corner (posterior along proximal tibia), superficial MCL (distal to deep portion), and PCL release that can assist in balancing.

  • Valgus deformity less common in TKA. Tight laterally and lax medially. Resect osteophytes, meniscus, lateral capsule, and IT band. IT band can be released with Z-plasty, off of Gerdy’s, or via “pie-crusting” (puncture hole) technique. Next can release popliteus (if tight in flexion) or lastly LCL if planning to use constrained prosthesis.

Sagittal Balancing: Balance Components in Flexion and Extension

  • Modifying the tibia (via tibial cut or poly thickness) affects both flexion and extension gap. Distal femur cut affects extension gap, and femoral component size affects flexion gap.

  • Tight in flexion and extension: Cut more tibia.

  • Loose in flexion and extension: Thicker poly or thicker tibial metal component.

  • Tight in flexion only: Decrease size femoral component, release PCL, resect posterior slope tibia, and resect more posterior femoral condyle and posterior capsular release.

  • Loose in flexion only: Increase femoral component size and augment posterior femur.

  • Tight in extension only: Cut more distal femur and release more posterior capsule.

  • Loose in extension only: Augment distal femur.

  • Combination tight/loose in flexion/extension: Use mix of the balancing techniques above.

  • Anticoagulation: Risk of DVT/PE elevated postoperatively so anticoagulation is needed to prevent thrombosis. Multiple options and widely debated in the literature. Established orthopedic guidelines have recommended one of the following: acetylsalicylic acid (aspirin) and sequential compression devices (SCDs), enoxaparin, or warfarin for anticoagulation. Research into newer medications (such as direct thrombin inhibitors) for anticoagulation is ongoing.

Complications of TKA

Patellar maltracking, arthrofibrosis/stiffness, periprosthetic infection, wound complications, periprosthetic fracture, heterotopic ossification, instability/poorly balanced, nerve injury, thromboembolic disease, late loosening, wear problems, or premature failure of components.

Patellar Maltracking

  • Most common reason for revision following TKA. Ensure appropriate imaging lateral and merchant (sunrise) x-rays to assess patellar component. See Q angle section above for discussion of technical considerations to avoid maltracking.

Stiffness/Arthrofibrosis

  • Flexion <90°. If identified within the first 12 weeks post-op, can perform manipulation under anesthesia (MUA) to improve ROM. Caution with MUA as overly aggressive manipulation can lead to fracture or failure of the extensor mechanism.

Periprosthetic Infection

  • Chronicity of infection guides treatment. Aspiration and culture gold standard for diagnosis. Synovial WBC > 1100 cells/mL and/or PMNs >64%. Current gold standard of management requires explantation and two-stage exchange with an antibiotic spacer targeted to culture results. In limited cases where infection is acute (less than 4 weeks post-op), can attempt irrigation and debridement with polyethylene exchange and retention of components.

Poorly Balanced/Unstable TKA

  • Flexion/extension, varus/valgus, global instability. Responsible for up to 20% of revisions. See balancing section above for technical considerations to properly balance a TKA.

Nerve Injury

  • Peroneal nerve is most commonly injured. Pre-op risk factors include valgus knee or flexion deformity. Fifty percent improve over time; after 3 months of nerve disruption, get EMG.

Revision

  • Indications: Patellofemoral maltracking, loosening (tibial > femoral), poorly balanced, instability, infection, poly wear, fracture, and component failure. Further discussion of revision beyond the scope of this review.

General Tips for Medical Students

  • TKA surgery has a large number of steps done quickly with extensive instrumentation. Find out what type of implant your team will be using, and typically you can get access to free technique guides online for the specific components and instrumentation you will be using. This allows you to familiarize yourself with the steps and the specific components you will see in the case. Repetition will breed familiarity. Knowing how to properly balance a TKA is critical.

    As with any rotation, know your patients! Keeping track of postoperative labs, vitals, anticoagulation, and other protocols will help you understand your patients better. Getting in early and gathering info on the patients before the resident shows up and giving them a sheet summarizing the patients on the floor will help you stand out and make a valuable contribution to the team. Ask a resident or attending surgeon to review how to systematically evaluate pre- and post-op imaging for TKA.