When an existing knee replacement has failed, requiring revision, careful pre operative assessment and intra operative technique is necessary to provide the best likelihood of restoring the lower limb alignment and pain free range of motion to achieve the optimum result.
Knee replacements can fail for a number of reasons but most commonly due to loosening of components over a long period of time, resulting in a recurrence of pain and swelling. Other reasons for revision of a knee replacement include; stiffness, instability, malpositioning of the original implants, patella dislocation, arthritis of a retained native patella, fracture, and infection (see Septic Revision Replacement – Infected Revision TKR). The commonest cause for loosening is where microscopic particles of plastic are generated as a result of the femoral component wearing on the tibial polyethylene liner, and these loose particles are attacked by the patient’s immune system, and are consumed by the body’s white cells. After the white cells have digested the plastic particles, they die and release their enzymes into the knee joint, resulting in inflammation of the knee’s synovial tissue or synovitis and also, leeching of the enzymes into bone at the interface where the implant attaches to the bone, with initial minor resorption of bone under the implants, but over time, cyst formation and bone stock loss and eventual loosening of components, with this process known as “Aseptic Loosening”.
When a decision is made to revise a knee replacement for loosening, the possibility of infection must first be excluded (See Septic Revision Replacement – Infected Revision TKR) usually with blood tests, a series of special scans (bone scan and labeled white cell scan), possibly an aspiration or biopsy of the knee joint. Once infection has been excluded, consideration is given as to why the knee replacement has failed, looking at the overall alignment in the frontal (coronal), side on (sagittal) and cross sectional (axial) planes, using a CT scan (Perth CT Protocol), consideration of the implant used, whether it was cemented or uncemented, as some implants have been shown to have a very high failure rate irrespective of how they were positioned and fixed to bone. Patient factors including osteoporosis which may require bone densitometry (DEXA Scan), Vitamin D and calcium levels, and the degree of use (volume of walking, cycling, squatting, kneeling, higher impact recreational activities such as tennis, squash or jogging) and trauma (falls, jumping, vehicular accidents). When a knee is being considered for revision for reasons other than loosening, the possible causes must also be considered both pre and intra-operatively to maximise the likelihood of successfully overcoming the underlying problem. If revising for deformity (malalignment) the pre-operative and initial post-operative X-rays will usually give a good idea about the adequacy of the original surgery, and whether or not it is a persistence of the original deformity or a new problem. Sometimes deformity will be due to a combination of bony and soft tissue factors such as ligamentous or capsular tears leading to both deformity and instability, or ligamentous or capsular contractures leading to both deformity and stiffness.
When a knee is being considered for revision for stiffness, the results are usually less reliable than for loosening of implants, pain or instability, depending upon the duration and severity of the stiffness, as the surrounding quadriceps and hamstring muscles develop contractures after a while which may be difficult to overcome. Causes for stiffness in a knee replacement include, early post operative adhesion formation and arthrofibrosis, which may occur if there was inadequate post-operative pain relief or swelling management, and once established, excision of scar tissue and synovitis may be necessary in order to regain range of motion. More commonly, the bony gaps between the ends of the femur and tibia at the time of the initial knee replacement are not evenly matched in extension and flexion or possibly having retained bone spurs or “osteophytes” which can lead to impingement and difficulty in getting the knee out straight or bent up, and requires careful consideration of the sizing and positioning of the new implants and removal of excess bone to help achieve balanced flexion and extension gaps. There are a number of soft tissue contractures which can restrict extension, (tight posterior capsule, posterior cruciate ligament, iliotibial band and/or contracted medial collateral ligament, gastrocnemius), and restrict flexion (popliteus tendon laterally, semimembranosus tendon and posteromedial corner medially). Other bony considerations include “overstuffing” of the anterior recess with under-resection of the patella and/or oversizing and excessive forward migration of the femoral component making it difficult to achieve full flexion and a relatively uncommon cause of stiffness, is heterotopic ossification, a condition of excessive bone formation within the knee joint capsule, quadriceps and surrounding tissues, which usually needs removal of bone from the soft tissue and consideration of radiotherapy and or non-steroidal medication (Indomethacin) to decrease the likelihood of recurrence.
When a knee is being revised for instability, it is important to establish whether the knee was always unsteady after replacement or whether it was initially stable and later became unsteady, in which case, a careful assessment of which restraints have failed, if any, or whether the anatomical restraints (MCL, LCL, PCL, posterior capsule, extensor mechanism) are present but not adequately tensioned. If the primary knee replacement used a cruciate retaining (CR) polyethylene liner with preservation of the posterior cruciate ligament (PCL), and the instability is due to a PCL rupture, this can lead to instability in the frontal plane, most noticeable when walking down stairs or stepping down off a curb, and can be corrected by simply replacing the tibial polyethylene insert to a posterior stabilised (PS) alternative, with either a vertical post engaging on the femoral components horizontal cam, and if no femoral component cam exists, usually a deep dished tibial polyethylene option will exist with a large anterior lip, to provide greater stability without need for a femoral component revision. If a rupture of a reconstructable ligament, such as the medial or lateral collateral ligament (MCL, LCL) has occurred recently, and the TKR is otherwise stable and well functioning, a period of bracing in a hinged range of motion (ROM) brace, with or without a ligamentous repair or reconstruction may be appropriate. If the injured ligament is under excessive load, such as a medial collateral ligament (MCL) in a physiologically aligned valgus knee, then the knee may need to be augmented with an “internal brace” using a constrained polyethylene liner which may require revision to a new femoral component to accommodate the more stable liner. If the instability has been longstanding, or the knee is significantly malaligned or malrotated, or the flexion and extension gaps widely mismatched, or the soft tissue tension inadequate due to implant subsidence and loss of “joint height”, or the implants worn or loose, then revision of the TKR is integral to achieving a stable well functioning knee replacement and the abovementioned salvage options are not likely to be worthwhile. After correction of the malalignment, malrotation, flexion and extension gap mismatches and restoring joint height, a reassessment of soft tissue balance and stability will determine the additional steps necessary to achieve a stable and well functioning knee. If the instability is due to significant shortening of the distal femur or proximal tibia from implant subsidence (bony collapse or fracture) but with good MCL and LCL integrity, then where possible reconstruction with structural bone grafting (allografts) or metallic augments and stemmed revision implants is likely to achieve a stable knee. If the soft tissue envelope is compromised and non-reconstructable but there is adequate bone stock a resurfacing type revision knee replacement with a rotating hinge type articulation may be appropriate and finally, if the degree of bone loss is extensive and soft tissue integrity also compromised, consideration of an “endoprosthesis” with possible reattachment and re-tensioning of the collateral ligaments or extensor mechanism but more likely combined use of an endoprosthesis with a rotating hinge articulation. The rotating hinge device and endoprosthesis is usually reserved as a last resort for situations where there is significant soft tissue and or bony compromise as depending on the degree of bone stock loss, few options exist for redo revision after this implant fails.
If a knee replacement is being considered for revision for pain, but the knee is not obviously loose or infected, great care must be taken to exclude other sources of pain, to avoid performing a revision knee replacement with persistence of the pain. A detailed assessment of the patient and the nature of their pain is essential, with the key being whether the pain is well localised and reproducible or not, and if so, where, and whether it is superficial or deep. If superficial and localised – a nodule on the end of a previously cut superficial nerve (cutaneous neuroma) or vascular malformation (capillary haemangioma) may account for well localised findings and can usually be treated with local ablative methods. If superficial and localised over a broader dermatomal area, ie. lateral thigh and calf and sole of foot, then consideration of a proximal or central cause such as referred pain arising from an arthritic hip joint which usually does not extend below the knee or a lumbosacral sciatica with nerve root impingement and referred pain into the buttock, thigh, knee, calf and foot depending on which nerve roots are involved. This will usually become apparent on examination of the lower back and hip joint and lower limb motor and sensory function and deep tendon reflexes. Confirmation of referred source of pain, through alleviation of symptoms by injection of local anaesthesia under X-ray guidance, allows for that particular pathology to be addressed (by a hip or spine surgeon as the case dictates). If deep and localised – possible anatomical causes include; medially – a pes bursitis from overactive hamstrings trying to stabilise an unsteady knee or due to excessive activity/exercise, laterally – an inflamed iliotibial band (ITB), popliteus tendon or biceps femoris or proximal tibiofibular joint arthritis and / or ganglion cyst, laterally, anteriorly – a patella or quadriceps tendinitis/tendinosis , and posteriorly – a gastrocnemius tendinitis/tendinosis or Bakers cyst at the back of the knee. Advanced imaging with an MRI is usually not possible once a knee replacement is in situ, so clinical examination is essential and confirmation with ultrasound and or ultrasound guided injections of local anaesthetic from a diagnostic viewpoint may be useful. If superficial and generalised, consideration of a complex regional pain syndrome (CRPS) once referred to as reflex sympathetic dystrophy (RSD) needs to be considered; with symptoms including a combination of; swelling, hyperaesthesia or hypersensitivity (allodynia) to touch, severe pain (neurogenic inflammation), and change in skin colour (red, white and blue), temperature (cold, warm, hot) and moisture (dry, sweaty) due to vasomotor dysfunction. The diagnosis is a clinical one, but often will have a characteristic appearance on bone scan (uniform widespread dense tracer uptake on the limb involved) and warrants avoidance of any surgery or invasive intervention and referral instead to a neurologist for management. If deep and generalized – consideration of synovitis, from polyethylene wear disease, or an underlying rheumatological condition (rheumatoid arthritis, ankylosing spondylitis, inflammatory bowel disease) or infection (See Septic Revision Replacement – Infected Revision TKR). In carefully considered cases, a series of blood tests and a sterile aspiration and or arthroscopic biopsy may shed some light on the underlying pathology and definitive management.