Instability accounts for approximately 20% of revision total knee arthroplasty (TKA) operations, however, diagnostic tests remain relatively subjective. The aim of this examination was to evaluate the feasibility of using pressure mat analyses during functional tasks to identify abnormal biomechanics associated with TKA instability. Five patients (M = 4; age = 69.80±7.05 years; weight = 79.73±20.12 kg) with suspected TKA instability were examined compared to 10 healthy controls (M = 4; age = 44.6±7.52 years; weight = 70.80±14.65). Peak pressure and time parameters were measured during normal gait and two-minute bilateral stance. Side-to-side pressure distribution was calculated over 10-second intervals during the second minute. Mann-Whitney tests compared loading parameters between groups and side-to-side differences in TKA patients (significance level = p<0.05). Pressure distribution was expressed relative to bodyweight. Notable differences were seen during bilateral stance. Uneven side loading was greater – favouring the non-operated limb – in TKA patients during bilateral stance compared to controls. This was significantly different at 30s (p=0.0336) and 60s (p=0.0336). Gait analyses showed subtle pressure distribution differences in unstable TKA patients. Stance time was indifferent. TKA patients tended to exhibit longer heel contact time (0.76s vs. 0.64s and reduced weight acceptance (50.75% vs. 56.75%) on the operated limb compared to the non-operated limb. Side-to-side differences in peak toe-off forces were significantly more pronounced in TKA patients versus controls (9.25% +/− 1.5% vs. 1.67% +/−5.79%; p=0.0039). Conclusion: This feasibility work demonstrates subtle differences in limb loading mechanics during simple clinical tests in unstable TKA patients that might be invisible to the naked eye. In the long-term, pressure analyses may be a useful diagnostic tool in identifying patients that would benefit from revision surgery for TKA instability

Instability accounts for approximately 20% of all revision total knee arthroplasty (TKA), however diagnostic tests remain crude and subjective. The aim of this examination was to evaluate the feasibility of pressure mat (SB Mat, TekScan) analyses of functional tasks to differentiate instability in a clinical setting. Five patients (M = 4; age = 69.80±7.05 years; weight = 79.73±20.12 kg) with suspected TKA instability were examined compared to five healthy controls (M = 1; age = 46.80±7.85 years; weight = 71.54±16.17 kg). Peak pressure and time parameters were measured during normal gait and two-minute bilateral stance. Side-to-side pressure distribution was calculated over 10-second intervals during the second minute. Pressure distributions were expressed relative to bodyweight (%BW). T-tests compared loading parameters between groups (significance level = p<0.05). Analyses showed subtle differences in pressure distribution in unstable TKA patients versus healthy controls. Stance time during gait was indifferent. TKA patients tended to exhibit longer heel contact time (0.76 vs. 0.64 sec) and reduced weight acceptance (50.75% vs. 56.75%) on the operated versus non-operated limb. Side-to-side differences in toe-off forces were significantly more pronounced in TKA patients versus controls (9.25% vs. 3.75%; p=0.0088). Uneven loading was significantly greater – favouring the non-operated limb – in TKA patients during bilateral stance compared to controls (p<0.05). This feasibility work demonstrates subtle differences in limb loading and biomechanics during simple clinical tests in unstable TKA patients that might be undetectable to the naked eye. Pressure analyses may therefore be a useful diagnostic tool. These findings warrant further investigation

Background. The aim of this study was to evaluate causes and results of revision surgery in unstable total knee arthroplasties. Methods. We retrospectively reviewed 24 knees that underwent a revision arthroplasty for unstable total knee arthroplasty. The average follow-up period was 33.8 months. We classified the instability and analyzed the treatment according to its cases. Stress radiographs, postoperative component position and joint level were measured. Clinical outcomes were assessed using the score and range of motion of the Hospital for Special Surgery (HSS). Results. Causes of instability included coronal instability with posteromedial polyethylene wear and lateral laxity in 13 knees(Fig. 1) and coronal instability with posteromedial polyethylene wear in 6 knees(Fig. 2), coronal and sagittal instability in 3 knees including post breakage in 1 knee(Fig. 3), global instability in 1 knee(Fig. 4) and flexion instability in 1 knee. Mean preoperative/postoperative varus and valgus angles were 5.8°/3.2° (p = 0.713) and 22.5°/5.6° (p = 0.032). Mean postoperative α, β, γ, δ angle were 5.34°, 89.65°, 2.74°, 6.77°. Mean changes of joint levels were from 14.1 mm to 13.6 mm from fibular head (p = 0.82). The mean HSS score improved from 53.4 to 89.2 (p = 0.04). The average range of motion was changed from 123° to 122° (p = 0.82). Conclusions. Revision total knee arthroplasty with or without a more constrained prosthesis will be a definite solution for an unstable total knee arthroplasty. The solution according to the causes is very important and seems to have a chance of avoidance of unnecessary over-constrained implant selection in revision surgery for total knee instability

Instability is a common indication for early revision after both primary and revision total knee arthroplasty (TKA), accounting for up to 20% in the literature. The number of TKAs performed annually continues to climb exponentially, thus having an effective algorithm for treatment is essential. This relies on a thorough pre- and intra-operative assessment of the patient. The underlying cause of the instability must be identified initially and subsequently, the surgeon must be able to balance the flexion and extension gaps and be comfortable using a variety of constrained implants. This review describes the assessment of the unstable TKA, and the authors’ preferred form of treatment for these difficult cases where the source of instability is often multifactorial. Cite this article: Bone Joint J 2016;98-B(1 Suppl A):116–19

Background:. To evaluate causes and results of revision arthroplasties in unstable total knee arthroplasties. Methods:. We retrospectively reviewed 24 knees that underwent revision arthroplasty for unstable total knee arthroplasty from December 2004 to December 2010. The mean age was 71.0(range, 54–85) years and the average follow-up period was 33.8 months (range, 6–70). The mean interval between the primary TKA and revision TKA was 82.5 months (range, 14–228). We classified the instability and analyzed the treatment according to its cases. Stress radiographs, postoperative component position and joint level were measured. Joint line position was measured using the fibular head as the reference point. Clinical outcomes were assessed using the Hospital for Special Surgery (HSS) score and range of motion. Wilcoxon sign rank test was employed for statistical analysis, and when p-value was over 0.05, it was analyzed as having statistical significances. Results:. Causes of instability included coronal instability with medial laxity in 13 knees (Fig. 1) and with polyethylene wear in 6 knees, coronal and sagittal instability in 3 knees including post breakage in 1 knee (Fig. 2, Fig. 3), global instability in 1 knee, and flexion instability in 1 knee. Mean preoperative/postoperative varus and valgus angles were 5.8°/3.2°(p = 0.713) and 22.5°/5.6°(p = 0.032). Mean postoperative α, β, γ, δ angle were 5.34°, 89.65°, 2.74°, 6.77°. Mean change of Joint levels were form 14.1 mm to 13.6 mm from fibular head (p = 0.82). The mean HSS score improved from 53.4 to 89.2(p = 0.04). The average range of motion was changed from 123° to 122°(p = 0.82). Conclusions:. Revision total knee arthroplasty with more constrained prosthesis was a very effective. The solution according to the causes is very important in revision surgery for unstable arthroplasty

Stability after TKA is essential for knee function and patient satisfaction. Stability may be marginally more important even than alignment because “stability” means there will be ONE alignment, whereas INSTABILITY means there will be many alignments of the joint, usually the worst one for any loading pattern. Whereas alignment results from the orientation and size of implants, stability depends on all of these, plus soft tissue integrity and in many cases, surgical alteration. Ligament releases (and rarely reconstructions) will certainly be required if alignment is changed with the arthroplasty. Instability may be a subtle or flagrant problem. The “Instabilities” are: . i. Varus- valgus. ii. Plane of motion- Flexion. iii. Plane of Motion-Extension. Varus-valgus instability is the prototype and while it may originate exclusively from the failure of soft tissue, knee alignment and dynamic forces outside the knee joint such as hip abductor dysfunction, scoliois and tibialis posterior rupture may be implicated. A comprehensive approach will be needed. Flexion instability, most simply stated results from a flexion gap that exceeds the dimensions of the extension gap. It will result most commonly after surgery for the patient with a fixed flexion contracture whose knee extends fully because a relatively thin polyethylene insert has been selected. So-called “mid-flexion” instability (implying stability in extension and flexion) has not yet been thoroughly characterised. Extension instability includes all failures of the extensor mechanism (rupture, maltracking and weakness) which are better characterised as “buckling” under a separate topic. Recurvatum has received little attention but can generate the most destructive forces leading to knee arthroplasty failure. In general begins as a compensatory mechanism for relative extensor weakness. All treatment of the unstable TKA must characterise the mode(s) of failure above and correct the underlying cause. Surgical technique will be extremely important, followed eventually by implant selection

The management of patients with a painful total knee replacement requires careful assessment and a stepwise approach in order to diagnose the underlying pathology accurately. The management should include a multidisciplinary approach to the patient’s pain as well as addressing the underlying aetiology. Pain should be treated with appropriate analgesia, according to the analgesic ladder of the World Health Organisation. Special measures should be taken to identify and to treat any neuropathic pain. There are a number of intrinsic and extrinsic causes of a painful knee replacement which should be identified and treated early. Patients with unexplained pain and without any recognised pathology should be treated conservatively since they may improve over a period of time and rarely do so after a revision operation.