Total Knee Replacement

  • The primary stabilisers about the knee are ligaments and muscles, as the bony contours offer little resistance to displacement
  • Walking on level ground, the joint is subject to 3 times body weight and going up stairs 4 times body weight, due to the effect of muscle action
  • The load to the tibia is not distributed evenly, with the medial tibial plateau taking greater load than the lateral

Indications for TKR

TKR is indicated if there is disabling functional impairment and other treatment modalities have failed

Primary objective

  • To relieve pain
  • To provide motion with stability
  • To correct deformity

Common conditions

  • RA; regardless of age, including JCA
  • OA
    • Consider age, weight, occupation, sex, activity level
    • Avoid in :
      • <60 y/o
      • Manual labourers
      • High demand patients
      • Obese
    • Post traumatic OA : a rare indication for TKR in the younger patient
  • Failure of HTO
  • Patello-femoral OA : rare in isolation. if so, the best treatment in the older patient is TKR
  • Neuropathic joint : controversial. Insall believes it is feasible as long as correct alignment and stability are achieved

Contraindications for TKR

  • Quadriceps weakness; arthrodesis is a better choice
  • Active sepsis
  • Neuropathic joint (see above)
  • Stable arthrodesis 
    • Prospects of a successful arthroplasty are not good
    • Constrained prosthesis is usually required
    • In the event of failure re-arthrodesis may not succeed
  • Genu Recurvatum 
    • Associated with muscular weakness or paralysis 
    • Is likely to recur following TKR 
    • Places stresses on the prosthesis which increase likelihood of loosening
  • Relative contraindications
    • Young age
    • Recent sepsis or osteomyelitis
    • Peripheral vascular disease
    • Non-ambulatory patient

Investigations

  • Rule out any hidden infection; e.g. UTI
  • X-Rays
    • Full-length standing radiographs
    • Standing lateral in extension
    • Flexion lateral in 90-100 degrees
    • Objectives
      • Accurately assess anatomic / mechanical axes
        • Defines femoral entry point and cutting angles
        • Most important in :
          • Very tall
          • Very short
          • Deformed bones
      • Look for any deformity
      • Assess bone stock

Technique

Exposure

  • Routine approaches
    • Medial parapatellar cutaneous
    • Anterior midline
    • Medial parapatellar tendo-capsular incision is usual
  • Alternative approaches
    • Subvastus (Southern) approach : vastus medialis is reflected laterally
      • Preserves extensor mechanism and supreme geniculate artery
      • Allows faster rehab program
    • Lateral approach : sometimes used in the valgus knee
  • When there is previous incision
    • Try to incorporate it into new incision
    • If not possible, it is either parallel to the new incision or not
      • Cross it at 90°, if not parallel to new incision
      • Keep a 7 cm distance, if parallel to new incision
  • Difficult exposures
    • When exposing a stiff or ankylosed knee, the standard exposure will risk damage to the tibial tubercle
    • Options include:
      • Quadriceps turndown : a narrow, inverted, distally based V incision, extended until the patella can be everted
      • Rectus Snip : like standard approach, but incision is continued laterally across the tendon at the apex of the quadriceps tendon
      • Tibial tubercle osteotomy : a long segment 3 – 6 cm in length is elevated. Should be fixed back with screws

Limb alignment

  • Varus or valgus malalignment is associated with increased component loosening
  • Aim for mechanical axis passing through middle of proximal tibia
  • Cut proximal tibia 90 to its longitudinal axis
  • Cut distal femur in 5-7 valgus
  • Both extra- and intra-medullary guides are available
    • Intramedullary guides are more accurate for femoral cuts
      • Its use can lead to significant malalignment when there is tibial or femoral bowing
    • Extramedullary can be used for
      • Tibial cuts
      • Occluded femoral canal
      • Deformities; e.g. femoral bowing
  • Severe extra-articular deformities (e.g. malunion or bowings) may need simultaneous corrective osteotomy and stabilisation, if have caused :
    • > 10 degrees valgus or varus
    • > 20 degrees extension or flexion deformity

Ligament balance

  • Both MCL and LCL should have equal tension through ROM
  • Ligament balance provides optimum knee function and minimal wear
  • If ligament balancing is necessary, it is best done by appropriate releases rather than advancements
  • If fixed deformities greater than 15 are present, the best results are obtained by excision of the PCL and use of a cruciate substituting prosthesis
  • Varus release
    • Remove medial osteophytes from the proximal tibia
    • Raise a sleeve of soft tissue from the proximal tibia and let it slide proximally
    • Order of release :
      • Medial capsule
      • Deep MCL; subperiosteally
      • Postero-medial corner
      • Superficial MCL and pes anserinus
        • Total release of superficial MCL should be avoided
        • Aggressive medial release leaves the joint incompetent and needs a constrained prosthesis
      • PCL
  • Valgus release
    • Risk of peroneal nerve palsy
    • First remove lateral osteophytes
    • Order of release:
      • Lateral capsular attachments to the tibia 
      • ITB :
        • If tight in extension
        • Either release from Gerdy’s tubercle or Z-lengthening
      • Popliteus tendon if tight in flexion
      • Both ITB and popliteus tendon if there is > 15° valgus
      • LCL 
        • Rarely required
        • Constrained prosthesis strongly recommended
      • Postero-lateral capsule ± PCL 
      • Lateral head of gastrocnemius
      • Lateral intermuscular septum with vastus lateralis ± biceps tendon
    • Lateral release may be coupled with medial soft tissue advancement
      • If so, lateral release is restricted to IBT and postero-lateral joint capsule
      • Needs post-operative splinting for 3 – 4 weeks

Flexion and extension gap

  • Ensure flexion and extension gaps are equal
  • General rules :
    • If gap problem is symmetrical (loose/tight in both flexion and extension), adjust tibia
    • If gap problem is asymmetrical (loose/tight in only one position), adjust distal femur
  • Gap problems can be addressed with one or a combination of these strategies :
    • Changing thickness of polyethylene insert
    • Changing femoral component size
    • Tibial slope modification
    • Soft tissue recess/release
    • Adding augmentation components
    • Selective resection of bony ends
  • Flexion gap modification
    • Remove posterior osteophytes
    • Resect more bone off the posterior aspect of distal femur
    • Elevate posterior capsule ± PCL release
    • Posterior capsule can be divided transversely near its origin from the femur
    • For minimising bone resection, divide or strip posterior capsular attachment to the femur, which may include both heads of gastrocnemius

Component Orientation

  • Sagittal
    • Most systems aim for 3° – 7° posterior tibial slope
    • To achieve this, tibial component should be perpendicular to proximal tibial trabeculae in lateral view
    • Too much femoral component extension may cause notching of anterior femur and increase risk of fracture
    • Too much femoral component flexion causes FFD
  • Axial (transverse)
    • Align tibial component to point to medial 1/3 of tibial tubercle
    • IR of the tibial component causes
      • Lateral displacement of the tibial tubercle
      • Increased Q angle
      • Patellar maltracking
    • IR of femoral component causes
      • Patellar maltracking
      • Asymmetrical flexion gap with
        • Tight, painful medial flexion gap
        • Flexion instability because of loose lateral flexion gap
    • Aim for 3° ER of femoral component to :
      • Improve patellar tracking
      • Equalise collateral structure balancing, as the lateral structures are slightly lax in flexion
  • Joint line
    • Level of joint line should be maintained as close to normal as possible
    • Generally, 8-10 mm of proximal tibia and 6-8 mm of distal femur is removed
    • Any change in joint line will alter
      • Ligamentous balance
      • Patellar height
    • This is more important in PCL retaining design, where tension of the retained PCL must be normal to ensure full ROM and even pressure transfer throughout the ROM
    • Small defects < 10 mm deep are filled with cement
    • Bone grafting of tibial plateau is indicated if
      • > 50% of the plateau is defected
      • Cement depth would be > 5 mm in large defects
      • Cement depth would be > 10 mm in small defects

Patellar resurfacing

  • 25% of un-resurfaced patellae will have post-op patellar problems
  • Patellar height should be restored with inserted component
  • If the remaining thickness of patellar bone after resection would be < 10 mm, options are :
    • Leave the patella non resurfaced
    • Perform patellectomy
  • Pitfalls and consequences :
    • Under-resection : increased wear
    • Over-resection : patellar fracture
    • Lateralisation of patellar component : maltracking
    • Asymmetrical cut : patellar tilt
  • Metal-backed patellar components have increased wear rate and should not be used



Patellar tracking

  • Usual causes of patellar maltracking
    • Malrotation of tibial or femoral components
      • CT is the evaluating test of choice
      • Compare component axes with femoral epicondylar and tibial tubercle axes
    • Medialisation of femoral component
    • Lateralisation of patellar component
    • Raised joint line
    • Residual genu valgus
  • Perform lateral release if required

Component Design

There are 2 major designs

  • Non-constrained :
    • Common design used in primary TKR
    • Majority of non-constrained designs sacrifice ACL
    • There are limited designs which retain both ACL and PCL, but :
      • ACL is not functional in almost all knees requiring TKR
      • Even if functional, it is difficult to achieve ligament balance
      • Rarely used
    • Subtypes :
      • Rotating tibial platform
        • Also known as Mobile Bearing Tibia (MBT)
        • Tibial poly insert rotates on a polished tibial tray
      • Fixed tibial platform
        • PCL retaining
        • PCL substituting (posterior stabilised)
        • PCL sacrificing
          • The earliest design, sacrificing both ACL and PCL
          • Provided limited ROM of around 90°
          • The joint was sagittally unstable in flexion
  • Constrained : used in revision and occasionally in complex primary TKR
    • Hinged
    • Non-hinged

Advantages and disadvantages of different designs

  • Advantages of mobile bearing tibia
    • Better conformity reduces wear
    • Automatically corrects rotational deformity
  • Disadvantages of mobile bearing tibia
    • Needs careful balance of flexion-extension gap
    • Bearing spinout : rotational dislocation of bearing surface in flexed position
      • A unique problem of mobile bearing design
      • Presents with locking
      • Occurs when there is a loose flexion gap
      • Requires revision to address flexion gap
  • Advantages of PCL retaining
    • Restores near-normal knee motion, including axial rotation of the tibia and roll back of femur on tibia
    • Maintains proprioception of the PCL
    • Maintains load transfer by PCL
    • The best choice if ROM is expected to be >130°, to prevent ‘cam jump effect’ of PCL substituting design
    • In gait analysis studies, patients with retained PCL have a more normal stair climbing gait. No difference in level gait
  • Disadvantages of PCL retaining
    • Need to reproduce the preoperative joint line or get increased joint reaction forces, which leads to wear
    • More difficult collateral balancing
    • Partial release of the PCL may be necessary to achieve ligament balance
    • Exposure may be more difficult in PCL retention
  • Advantages of PCL substituting
    • Congruent poly reduces contact stress and wear
    • Improved flexion
    • Fixed roll back
    • Less sensitive to joint line changes
    • Corrects moderate deformity
    • The best choice in :
      • Patellectomised knee; gives more AP stability
      • Inflammatory arthritis; continued inflammation eventually compromises PCL
      • Previous PCL rupture
      • Flat tibial profile; decreased tibial slope limits flexion of a PCL retaining implant
  • Disadvantages of PCL substituting
    • Needs careful balance of flexion-extension gap
    • Patello-femoral clunk
    • Implant dislocation (cam jump)
    • Femur jumps over tibial cam in flexion, if flexion gap is loose
    • Still may occur in hyper-extension in a well-balanced knee
    • Loss of PCL proprioception
    • Femoral notch resection
  • Good results obtained in all types
    • PCL retaining : 90% good or excellent results after 5 – 9 years
    • Posterior stabilised : 83% good or excellent results after 8 years
    • PCL sacrificing :
      • All poly : 88% good or excellent results after 10-12 years
      • Metal-backed : 96% good or excellent results after 11 years

Fixation

  • Cemented tibial component
    • Remains the gold standard
    • Survival rates:
      • Posterior stabilised prosthesis : 90% at 15 year
      • All polyethylene tibial component : 97% at 10 year
      • Metal backed tibial component : 99% at 7 year
  • Cementless tibial component
    • More prone to failure than cemented
    • Probable mechanism of failure is insufficient initial stability for adequate bone ingrowth
    • Disadvantages :
      • Decreased longevity
      • Metal fatigue
      • Patellar component problems
  • Hybrid TKR
    • Cementless femoral plus cemented tibial components
    • Early results are equal to all cemented
    • 93% good or excellent results at 7 years

Polyethylene

  • Need to have minimal thickness of 8 mm
  • Metal-backed polyethylene
    • Allow more even distribution of forces
    • Polyethylene wear may occur between poly and metal backing
    • Polyethylene is usually thinner than all-poly components to prevent raising joint line or resecting too much of tibial bone
    • Wear and fatigue rates of polyethylene increase significantly only if <6 mm
  • Rate of wear is influenced by :
    • Limb alignment
    • Component design
    • Presence of metal backing
    • Poly thickness
    • Modularity
  • Less conforming surfaces (PCL retaining design) have more point loading and wear

Prognosis

  • 94% survival at 11 years for total condylar prostheses
  • Young patients : Short-term survival rate has been similar to older age groups
  • Obese patients :
    • Similar short-term survival rates
    • Higher incidence of patellar complications
  • Diabetic patients have increased :
    • Infection rate
    • Revision rate
    • Mortality rate
  • Hemophilic patients :
    • Good short-term results
    • 50% complication rate
      • Nerve palsy
      • Infection
      • Bleeding
    • Factor VIII levels should be maintained at 100%
    • Prophylactic complete synovectomy should be undertaken

References

  • Di Cesare : Knee. In Spivak JM, Di Cesare PE, Feldman DS : Orthopedics; A Study Guide; 1999
  • McPherson EJ : Adult Reconstruction. In Mark D. Miller : Review of Orthopedics; 2008
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