Total knee arthroplasty (TKA) is a viable treatment for symptomatic osteoarthritis of the knee refractory to conservative measures. In those with end-stage degenerative changes compromising the articular cartilage affecting multiple compartments of the knee, the literature has yet to identify a potentially viable alternative option for the regeneration of cartilage. Thus, TKA has demonstrated reproducible, long-term, successful results in such patients concerning outcomes of decreased pain and improved overall quality of life.
Recent estimates project that by the year 2030 there will be 3.48 million TKAs performed annually. Although it is an extremely common and increasingly routine surgery, attention to detail is critical during the procedure to ensure that a well-balanced and functional TKA is performed to mitigate the risks of implanting components that might otherwise be subject to increased wear and early failure. Even with appropriate technique, new technologic advances, and a better understanding of knee kinematics, approximately 1 out of 5 people that undergo a TKA will remain unsatisfied. There are numerous TKA designs and different levels of constraint that may be necessary for particular cases. Unicompartmental arthroplasty, cruciate retaining, and posterior stabilizing implants are typically used as potential index procedure options. However, in patients with significant varus/valgus instability, those undergoing revision surgery including component revisions, patients with pre-existing poor bone quality or in the setting of appreciable osseous defects, more constrained prosthetic components are given consideration. These include, but are not limited to semi-constrained, hinged, or distal femoral replacement options.
The knee is made up of 3 separate compartments:
The knee is typically described as a hinged joint; however, there are more complex and subtle motion and dynamic considerations. Physiologically, the knee also undergoes axial rotation and femoral "rollback" in deeper degrees of flexion. Additionally, terminal rotatory motion, known as the "screw home mechanism," occurs as the tibia externally rotates when the knee goes into terminal extension. The lateral tibial plateau is convex and sits more proximal than the medial tibial plateau which is concave. The medial femoral condyle is larger than the lateral side. 60% of the force through the knee joint occurs on the medial tibial plateau. Thus, the medial tibial plateau is of more dense bone, and this is also why the more common wear pattern seen in osteoarthritis occurs on the medial side.
The distal femur is in approximately 9 degrees of anatomic valgus relative to the joint line while the proximal tibia in 3 degrees of anatomic varus relative to the joint line.
Typical patellar Q angle is between 13 degrees and 19 degrees, with an increased Q angle increasing the risk of patellar maltracking and dislocation.
The most common underlying diagnosis and indication for TKA are end-stage, degenerative osteoarthritis of the knee, with approximately 94- 97% of knee replacements performed for primary or post-traumatic osteoarthritis. These patients must have degenerative changes with pain and limitation of function in the knee that has failed conservative and non-operative measures. Other underlying diagnoses that may be treatable with a knee replacement include rheumatoid arthritis, peri-articular fractures, or malignancy. However, patients with malignancy may commonly require megaprostheses.
There are few absolute contraindications for TKA including an active infection in the joint, and remote infection, or bacteremia. Relative contraindications include a significant vascular disease that may lead to poor wound healing and increased risk for infection. There is currently discussion and controversy in the literature as to risk stratification and increased complications associated with obesity. Patients with a BMI greater than 30 have an increased risk of infection and medical complications compared to non-obese patients. This risk further increases for those with a BMI greater than 40. Patients with BMI over 40 were found to have a statistically significant increase in superficial infection, deep infection, operative time, deep vein thrombosis (DVT), length of stay, renal insufficiency, reoperation, and wound dehiscence.
Each implant company has a specific system with trays that come with the necessary tools and trial components. Basic equipment that will be used in all cases and will improve the efficiency and safety of the case include:
In revision cases, may also require other equipment including:
Implant types have continued to evolve since the 1950s. Insall et al. initially described in the 1970s the different models of knee prostheses. He placed them into two categories; condylar replacements that spared ligaments, and hinged-type components that sacrificed the ligaments. Four models were described, with increasing complexity in each model:
In modern arthroplasty, most implants are a derivative of these models that were initially described. From least complex to most complex they include unicompartmental, cruciate retaining, posterior stabilizing, constrained non-hinged, constrained hinged prosthetic components.
Cruciate Retaining (CR)
The ACL is sacrificed, overall feasibility mandates a competent and functional PCL.
Can be used in those with mild varus/valgus deformity. Should be avoided in those with inflammatory arthritis due to increased risk for short-term or delayed rupture of PCL.
Posterior Stabilizing (PS)
Can be used in those with absent PCL, inflammatory arthritis, and may be beneficial in those with a previous patellectomy as it can add some anterioposterior stability that is absent due to the weak extensor mechanism. PS femoral prosthetic component contains a box in the femoral component with a post on the polyethylene liner that substitutes for the resected PCL.
Overall, multiple studies have not demonstrated a significant difference in function, satisfaction or implant survival between cruciate retaining and posterior stabilizing implants.
Varus-valgus constrained design is used in cases where there is LCL or MCL deficiency, moderate bone loss, or flexion gap laxity. This implant has a large tibial post with a deep femoral box.
Rotating hinge prostheses are used in complex revision arthroplasty cases with significant bone loss, ligamentous laxity, or in oncologic cases. Femoral and tibial components link with an axle and the tibial bearing can rotate around the tibial platform. This rotation allows for a lower constraint and therefore a decreased risk of aseptic loosening. Early implants were uniplanar without allowing rotation and had a high rate of aseptic loosening.
Total joint arthroplasty has transitioned from a fee for service to a bundled payment to incentivize quality of care over quantity of cases. In 2015, the Center for Medicare and Medicaid Services (CMS) announced the Comprehensive Care for Joint Replacement Model (CJR). Bozic et al. in 2014 demonstrated that 70% of the total cost was post-discharge and they have created CJR in hopes of stimulating a reduction in cost and complications. This effort has led to the development of standardized protocols and an interprofessional approach for patients undergoing knee replacement. At our institution, personnel integral to the success of a knee replacement include:
Numerous non-operative treatments are employed in the pre-surgical management of knee arthroplasty. Treatments include both pharmacologic and non-pharmacologic options. Multiple studies have investigated these different treatment options, and guidelines have been put in place by the American Academy of Orthopaedic Surgeons in the 2013 evidence-based guidelines, 2nd edition.
The AAOS recommends with strong evidence for low-impact aerobic exercises, neuromuscular education, and strengthening. They also recommend strongly for the use of NSAIDs and tramadol.
The AAOS recommends moderate weight loss in patients with a BMI greater than 25.
The AAOS recommends strongly against acupuncture, glucosamine/chondroitin, viscosupplementation.
The AAOS recommends moderate against lateral wedge insoles.
The AAOS cannot recommend for or against manual therapy (i.e., chiropractic, joint manipulation), physical agents including electrotherapeutic modalities, biologic injections, corticosteroid injections, valgus directing force brace, acetaminophen, opioids, or pain patches.
A review of the literature did not demonstrate a uniform protocol as to what pre-operative evaluation is necessary before knee replacement. A thorough history and physical exam are necessary. Co-morbidities, smoking status, alcohol consumption, and mental status should all undergo evaluation. The patient risk for development of thromboembolic events should be a consideration. However, typically, most patients will undergo evaluation and pre-operative clearance by their primary care provider and potentially cardiologist if significant cardiac co-morbidities are present. Other subspecialty pre-operative evaluations may be necessary depending on patients other co-morbidities (i.e., rheumatology, nephrology, neurology). Bernstein et al. developed a protocol that assessed patients for 19 different risk factors. Identifying these risk factors allowed for pre-operative intervention and these patients were found to have a statistically significant shorter length of stay, and lower average total direct variable cost. No difference was noted in patients 90-day readmission rate.
Pre-operative Surgical Planning:
It is essential to assess for previous surgical incisions, patients limb-lengths, limb deformity, the range of motion, ligamentous stability and gait. Patients neurovascular status should also be considered, Ankle-brachial index may be a prudent screening, and if less than 0.9, a vascular consultation should be obtained. Patients with pre-existing peripheral vascular disease should also have their PAD assessed.
Plain radiography typically provides sufficient detail, and further studies are typically not necessary. Weight-bearing AP, lateral and sunrise views should be routinely performed. AP hip to ankle x-rays allow for evaluation of extra-articular deformities and allow for assessment of the mechanical axis. Radiographic markers can be placed during imaging to allow for the use of templating software to assess for estimated implant sizing and positioning. A lateral patellar shift of more than 3mm was an independent risk factor for patellar maltracking. Images should be interpreted to assess for Insall-Salvati ratio, a pre-existing slope of the tibial plateau and coronal alignment. Custom implants are made based on CT or MRI sequences, and these studies should be performed when a custom implant is desired. Custom implants can be particularly helpful in patients with significant deformity where intramedullary or extra-medullary guides will not work, as well as in patients with pre-existing hardware.
Basic pre-operative labs should be performed such as CBC, BMP, HbA1c in diabetic patients. A goal of HbA1c of less than 7.0 is desirable; however, studies have shown that a HbA1c under 8.0 is acceptable to avoid excessive delay and complications in knee replacement. Many centers also perform pre-operative urinalysis and nasal swabs/decolonization in potential MRSA carriers. Other considerations include checking a total lymphocyte count (TLC) and serum albumin as these are markers of potential underlying malnutrition that may increase the risk of wound complications following the elective TKA procedure.
Patients are typically placed supine on the operative table; a general or spinal anesthetic is administered. A systematic review performed by Johnson et al. found no statistically significant differences between the two including mortality, surgical duration, or nerve palsy. Typically a standard midline incision with a medial parapatellar arthrotomy is used. However, other approaches include the lateral parapatellar approach, midvastus, and subvastus approach. More extensile approaches include the quadriceps snip, V-Y turndown, and tibial tubercle osteotomy. It is essential during the approach to maintain thick skin flaps and to respect the blood flow that comes from medial to lateral. Maintaining a small cuff is necessary during the arthrotomy to allow for adequate repair at the conclusion of the procedure. The medial soft tissues at the proximal tibia are skeletonized off of the bone, and a soft tissue release is performed medially. The same procedure is performed on the lateral side. However, most cases involve varus deformities, and a more extensive medial release is the choice. The infrapatellar fat pad can either be partially or completely excised. The medial and lateral meniscus, as well as ACL, will require excision. The PCL should also be sacrificed if a posterior stabilizing implant is desired. It is not necessary to resurface the patella in all cases. However, in patients with significant anterior knee pain or patellofemoral arthritis, it should be. It is worth noting that the revision rate and incidence of anterior knee pain are higher in those that do not undergo resurfacing. But those with resurfacing have a higher chance of complications such as tendon injury or fracture. Overall the patient satisfaction rates are equal between the two treatments.
The sequence of steps during knee arthroplasty will be dependent on the technique selected by the operative surgeon. These techniques include:
1) Measured Resection
2) Gap Balancing
Patellar maltracking is one of the most common complications encountered in knee arthroplasty. Iatrogenic causes of the complication can be avoided by:
A uniform strategy for wound closure does not exist, and many studies have found different materials to be superior. Typically a heavy ethibond suture or barbed suture is used for arthrotomy closure. Followed by absorbable suture or barbed suture is used for deep and superficial dermis. Skin closure can be with either a running non-braided absorbable suture with either dermabond/steri-strips or staples. Silver lined dressings can be used and should be maintained for several days without being removed until drainage has stopped. For complex revisions or in high-risk patients, incisional wound vacs or negative pressure dressings are also an option.
Periprosthetic fractures can occur in the distal femur, proximal tibia, or patella. They are most commonly located in the supracondylar region of the distal femur. The most common classification system used for periprosthetic fractures of the distal femur is the Lewis and Rorabeck classification. In this classification system, type-I is non-displaced with a stable component; type-II has more than 5mm of displacement or greater than 5 degrees of angulation with a stable component, and type-III has a loose component. There are several additional classification systems; All of which fail to classify intra-operative periprosthetic fractures. The anatomic location is typically used to describe these fractures. Risk factors that may predispose patients to periprosthetic fractures include:
Treatment options can include open reduction internal fixation, knee immobilizer, revision arthroplasty if components are loose, or distal femoral replacement, if there is not adequate bone stock or significant comminution, is present.
Understanding joint arthroplasty principles and increasing the quality and continuity of care is very important. As stated earlier, 3.48 million total knee arthroplasty's (TKA) will be performed annually by 2030. Currently, 20% of patients undergoing TKA remain dissatisfied for a variety of reasons. This dissatisfaction places a heavy burden and cost on the healthcare system. More importantly, due diligence is a priority and must take place before performing a TKA in every patient as a compromised or negative outcome results in a multitude of suboptimal downstream effects. Furthermore, as alternative payment models and bundled payment initiatives continue to move across the entire healthcare system, including the entire orthopedic subspecialty fields, all healthcare providers, hospitals, and healthcare facilities are under increasing pressure to provide high quality, value-based healthcare at all levels at an increasingly efficient manner.
The Bundled Payments for Care Improvement initiative (BPCI) by CMS has lead to systematic approaches by many institutions. Clinical care pathways (CCP) include an interprofessional teams such as nursing, physical therapy occupational therapy, anesthesiologists, orthopedic surgeons, dietitians, and general surgeons. In patients suffering from obesity who are at increased risk of complications, potential weight loss with the assistance of a dietitian can be achieved. If they are unable to lose the necessary weight, bariatric surgery is a consideration. Immediate access to physical therapy, joint education classes, and improved pain management through nerve blocks and multimodal therapies has led to decreased narcotic use and quicker mobilization. The implementation of these CCP's has shown improved patient-reported outcomes, early mobilization, reduced hospital length of stay and reduction in hospital costs. Appropriate pre-admission testing and evaluation by primary care physicians and cardiac optimization have reduced the length of stay, mortality, and intensive care unit admissions.- Level 3 evidence
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