Current Concepts in Knee Replacement
Total knee replacement (TKR) is one of the most successful medical innovations developed in the last century, can substantially improve patients quality of life and has been well validated.
The rate of TKR is increasing globally as a result of population growth, clinical success of joint replacement and changing demographic patterns interacting synergistically. Some 40% of 40 year olds will have radiographic evidence of osteoarthritis of the knee and approximately half of these will have symptoms.
Between 2005 and 2030, the prevalence of primary TKR and revision TKR procedures in the United States is predicted to increase 673% and 601% respectively. In Ireland, based on VHI statistics, TKR have increased in number by 173.4% between 1999 and 2004.
NICE guidance suggests: 'referral for joint replacement surgery should be considered for people with osteoarthritis who experience joint symptoms (pain, stiffness and reduced function) that have a substantial impact on their quality of life and are refractory to non-surgical treatment. Referral should be made before there is prolonged and established functional limitation and severe pain.' Younger age should not preclude joint replacement surgery; Duffy et al retrospectively reviewed fifty-two consecutive TKR’s that had been performed with a cemented press-fit condylar design in patients who were fifty-five years of age or younger. After an average duration of follow-up of twelve years the survival rate was 96% at ten years and 85% at fifteen years. Equally morbidly obese patients are frequently declined surgery and although there is an increase in suboptimal alignment, minor wound complications, and a slightly higher rate of late revision, the overall complication rate is low in this morbidly obese group. Additionally, 85% of the patients were satisfied with the outcome.
Occasionally there may be an indication to replace a knee because of progressive deformity and/or instability, and pain may not necessarily be the most significant factor. Patients expectations continue to rise and include faster recovery, less discomfort, no activity limitations and long-term durability of the joint replacement. This has driven research in musculoskeletal science resulting in better bearing materials and designs to reduce wear, greatly increase implant longevity and improve function. Recent trends have focused on improved rehabilitation and pain management to accelerate post-surgery recovery.
TKR was attempted in the 1860s where Fergusson reported performing a resection arthroplasty of the knee for arthritis and Verneuil performed the first interposition arthroplasty using joint capsule. Other substances included harvested muscle and fat, nylon and even pig bladders to cushion the knee joint and relieve pain!
The first artificial implants were tried in the 1940s but failed due to persistent pain or loosening as these primitive simple hinge designs didn’t account for the complexities of knee motion. In the late 60’s a joint which took into account the more complex movement of femoral condyles over the tibia was developed by Frank Gunston but failed through inadequate fixation of the prosthesis. The modern era of TKR really began in 1972 where John Insall designed a prosthesis – the total condylar knee - made of three components which would resurface all three surfaces of the knee - the femur, tibia and patella. They were each fixed with bone cement and the results were outstanding.
The success of knee replacement surgery has now equalled, and perhaps even surpassed, that of hip replacement with 90-95% success rate at 10 to 15 yrs.
As there are over 150 knee replacement designs on the market Orthopaedic Surgeons have large numbers of knee devices from which to choose. Published results of many knee implants offer little help to the surgeon wishing to make an informed choice. Most outcome research is short term, non-comparative and does not take into account case-mix of the surgeon. Many factors determine surgeon preference for an individual implant these include age, weight, level of activity, health, cost of prosthesis, their trainers, consultant colleagues, the desire to improve their own results and service delivery of the manufacturers. Another confounding factor is that knee devices with apparently good published results have been modified by the manufacturers and even minor modifications to design, material, surface finish, or fixation techniques can dramatically alter the performance of a knee replacement. There should be at least a 90% ten year survival for knee prostheses.
The most significant variations in TKR are between partial and total replacement, cemented and uncemented components, operations which spare or sacrifice the posterior cruciate ligament, resurfacing the patella or not, fixed or mobile bearings, gender specific knees and high flexion designs.
There are broadly speaking 3 types of knee replacement:
Partial Replacement : Unicondylar/Patellofemoral/Combination - Bicondylar
Unicondylar Knee Replacement (UKR)
Occasionly if only one part of the knee joint is severely worn then uni-compartmental or patello femoral replacement surgery is recommended rather than TKR.
While unicompartmental knee arthroplasty offers both the possibility of a more rapid recovery as well as a conservative approach to the treatment of knee arthritis universal acceptance of UKR has been tempered by concerns about inferior long-term outcomes and the inherent difficulties of performing the surgery accurately. UKR’s indications are very limited and therefore only account for approximately 8 % of all knee replacements. Patients that may not be eligible for a UKR include patients that have an inflammatory arthritis, have major deformities that can affect the knee mechanical axis, have neuromuscular disorders that may compromise motor control and/or stability, have any mental neuromuscular disorder, patients who are not skeletally mature, are obese, have lost a severe amount of bone from the tibia or have severe tibial deformities, have recurring subluxation of the knee joint, have untreated damage to the patellofemoral joint, have untreated damage to the opposite compartment or the same side of the knee not being replaced by a device, and/or have instability of the knee ligaments such that the postoperative stability the UKR would be compromised. Inclusion criteria are as follows: isolated medial joint line tenderness, isolated medial compartment radiographic changes, an intact anterior cruciate ligament, a flexion deformity of <100, a correctable varus deformity of <150, and a body mass index of <30. Although several studies have demonstrated ten-year survival rates of >90% after medial UKR’s, there remains a concern that certain designs or populations may experience unacceptably high early failure rates. While most recent data suggests that UKR in properly selected patients has survival rates comparable to TKR, most surgeons believe that TKR is the more reliable long term procedure.
The National Joint Register of England and Wales reported 71,527 primary knee replacements in 2008, 91% were of the total condylar type, 8% unicondylar and just over 1% patello-femoral replacements. The majority were cemented prosthesis. The overall revision rate following primary knee replacement was 0.7% (95% confidence interval 0.6% - 0.7%) at one year, 2.5% (2.4% - 2.6%) at three years and 3.7% (3.5% - 3.9%) at five years. The three year revision rate for UKR was 7.2% (6.6% -7.9%) and 8.3% for patello-femoral replacement (6.6% - 10.5%).
In men, the risk of revision in the first three years following surgery with a UKR was 2.5 times higher than with a cemented TKR, whereas in women with a UKR the risk was 3.7 times higher.
Unconstrained bicompartmental Knee Replacement.
This is the most common form of TKR. The lower part of the replacement knee joint is comprised of a flat metal plate and stem that implants in the tibial bone. This tibial tray can be either cobalt chrome alloy or titanium alloy. It can be fixed by either cement or bone “ingrowth”. Next, a polyethylene insert is clipped into the tibial tray to serve as the new knee bearing surface. The upper part of the replacement knee joint consists of a contoured metal shield that fits around the lower end of the thigh bone (femur). The inner surface can be fixed to the cut bone surfaces by the surgeon’s choice of bone ingrowth or bone cement. The outer surface of the contoured metal shield is shaped to allow the patella to slide up and down in its groove. The surgeon may choose to retain the patella or re-surface it. In this case a polyethylene button will be cemented in place.
Care should be taken when using the term "total knee replacement" as this implies that all articular surfaces in the knee have been replaced including resurfacing of the patella. The issue of patellar resurfacing remains controversial as there is no strong data to support resurfacing or non resurfacing.
Posterior Cruciate Ligament (PCL)-Retaining or Substituting
In total knee replacement surgery, the PCL can be kept or removed and this choice depends on the condition of the PCL, the type of knee implant or the type of surgery the surgeon likes to do. Each of these designs has advantages and disadvantages. Surgeon preference depends on his or her training and the clinical situation. PCL-Substituting knees (also called posterior stabilized knees) have a raised sloping surface or a polyethylene post that compensates for the missing PCL to give your knee more stability.
Knee replacements may be “cemented” or “cementless” depending on the type of fixation used to hold the implant in place. The majority of knee replacements are generally cemented into place. Cemented knee replacements have been used successfully in all patient groups for whom total knee replacement is appropriate, including young and active patients with advanced degenerative joint disease. 15 years of clinical reports support this conclusion.
Cementless fixation using a prosthesis with a textured, porous surface into which bone can grow may provide biologic fixation. That is, the bone grows into the prosthesis and holds it in place. Screws or pegs may also be used to stabilize the implant until bone ingrowth occurs. This may be more durable than cement used in the past.
The risks and benefits of components designed with the intention of promoting or accommodating high flexion after TKR is heavily debated. Though significantly more patients who receive the high-flexion design have >135° of flexion. I believe that early results may favour the use of femoral components with a high-flexion design, although long-term follow-up is necessary to report on implant survival. The early benefits of high-flexion knee designs may be more apparent as improved outcome tools are generated.
Constrained bicompartmental Knee Replacement.
These prosthesis are used principally in revision cases or when considerable bone loss (bone tumours) or collateral ligament damage is present. The joint works like a hinge and is much more prone to loosening.
Types of Polyethylene
Polyethylene used in knee joints can be non-crosslinked (sterilized by ethylene oxide gas or gas plasma methods) to moderately-crosslinked (gamma radiation sterilization). Generally speaking, increased crosslinking results in wear reduction, but there are design variables of the implant to consider as the fatigue strength of the polyethylene may reduce.
- 90% of patients have excellent or good outcomes following TKR
- Infection – 1-2%
Infection is the most devastating complication after TKR. Prophylaxis is the key and all patients should receive an intravenous broad spectrum antibiotic at induction of anaesthesia and two doses post operatively. The knee replacement should be performed in ultra clean air theatres and antibiotic impregnated bone cement should be used.
Prosthetic infections maybe categorised into four types.
- Type 1 (Positive intraoperative culture): 2 positive intraoperative cultures
- Type 2 (early postoperative infection): Infection occurring within first month after surgery
- Type 3 (acute hematogenous infection): Hematogenous seeding of site of previously well-functioning prosthesis
- Type 4 (late chronic infection): Chronic indolent clinical course; infection present for >1 month
The choice of treatment depends on the type of prosthetic infection.
- Positive intraoperative cultures: Antibiotic therapy alone
- Early post-operative infections: debridement, antibiotics, and retention of prosthesis
- Late chronic: delayed exchange arthroplasty. Surgical débridement and parenteral antibiotics alone in this group has limited success, and standard of care involves exchange arthroplasty
- Acute hematogenous infections: debridement, antibiotic therapy, retention of prosthesis
Venous Thromboembolism –50-70% with no prophylaxis!
Prophylaxis against venous thromboembolism after TKR continues to be a source of contention, particularly as hospital and nationwide initiatives and protocols are being implemented. The most referenced recommendation, by the American College of Chest Physicians, suggests that extended chemical thromboprophylaxis is mandatory and with the advent of new oral direct factor Xa inhibitors this is achievable. However, advances in rapid recovery protocols, early ambulation and multimodal analgesia also have a part to play.
Neurovascular Injury – 1%
Persistent pain or Stiffness – 5-10%
Fisher et al. identified TKR’s that were stiff or painful at one year after the index procedure. This group was compared with a matched control group of non painful knees with a well-functioning TKR that had had a similar preoperative range of motion to identify patient related factors that contribute to poor results after TKR. The authors identified female sex, a higher body mass index, previous knee surgery, disability status, diabetes mellitus, pulmonary disease, and depression as being significantly associated with the risk of having stiffness or pain at one year after surgery despite the presence of well-aligned, well fixed components. The importance of patient-related factors to the eventual outcome of TKR is clear, and these factors should influence preoperative counseling of patients awaiting TKR.
Prosthesis failure – 90% survivorship @ 10-15 yrs.
Primary knee replacement may fail between five and ten years but the majority fail after ten years. Studies emphasize the importance of limb alignment and polyethylene shelf age to the rate of polyethylene wear after TKR.
For best practice, patients should be followed up clinically and radiologically in the long term, however this is rarely possible with current resources. I believe that ideally a minimum requirement is an AP and Lateral weight bearing X ray at one year, and each five years thereafter. Failure from aseptic loosening of a knee replacement is often silent and the patient does not complain. Regular follow-up identifies the patient at risk of progressive failure. Exchange or revision operations should be planned and performed before massive bone destruction occurs, as delay may result in the need for much more extensive surgery which is more demanding of resources and has a greater risk of failure.
Minimally Invasive Surgery
Minimally Invasive Surgery was developed to reduce the size of the incision and limit damage to underlying structures however while these improvements have a real theoretical advantage, one must be cautious in its widespread use. At 3 months postoperatively there is no difference in the comfort and function of patients having conventional surgery and those having an MIS procedure. Also with limited access there is an increased risk of implant malposition that can affect the long term-success of the replacement. It is clear that the length of the skin incision has very little effect on postoperative recovery or blood loss after TKR. The use of smaller incisions may actually compromise wound-healing and can increase operative time which may increase the probability of contamination. My personal believe is to do the surgery as safely and as effectively as possible through the smallest incision possible.
Navigation and Robotics in Total Knee Arthroplasty
Meta-analyses of navigated TKR found that the risk of limb malalignment was slightly higher in the conventional total knee arthroplasty group. However, the clinical consequences of these small differences are not clear and the functional outcomes and complication rates were not clearly different.
The evolution of perioperative pain management and physical therapy protocols has had a profound impact on patient care after total knee arthroplasty. The multimodal pain protocol includes preemptive analgesia, avoidance of high-dose, short-acting intravenous narcotics and a periarticular injection of a combination of Ropivacaine, morphine, epinephrine and Ketorolac. The percentage of patients who are able to perform a straight leg raise on the first postoperative day is significantly higher in the multimodal pain protocol group than in the standard protocol group. Less narcotic consumption and fewer side effects as well as improved early functional recovery are common place.
TKR is successful in the majority of patients but patients’ expectations need to be managed appropriately. Future developments, such as navigation-guided surgery, better oral anticoagulants, enhanced kinematics, and wear-resistant bearing surfaces with better fixati on, promise a consistent evolution for the total knee replacement
Total Knee Replacement