Introduction

Periprosthetic medial tibial plateau fractures (TPF) are rare but represent a serious complication in unicompartmental knee arthroplasty (UKA). Most common treatment of these fractures is osteosynthesis with canulated screws or plates.

Periprosthetic tibial plateau fractures (PTPF) represent a rare but serious complication in unicompartmental knee arthroplasty (UKA). Although excellent long-term results have been reported with cemented UKA, surgeons continue to be interested in cementless fixation. The aim of the study was to compare fracture loads of cementless and cemented UKA.

Tibial components of the Oxford UKA were implanted in six paired fresh-frozen tibiae. In one set surgery was performed with cement fixation and in the other cementless components were implanted. Loads were then applied under standardised conditions to fracture the specimens.

Mean loads of 3.6 (0.7–6.9) kN led to fractures in the cemented group, whereas the tibiae fractured in the cementless group with a mean load of 1.9 (0.2–4.3) kN (p< 0.05).

The loading capacity in tibiae with cementless components is significantly less compared to cemented fixation. Our results suggest that, patients with poor bone quality who are treated with a cementless UKA are at higher risk for periprosthetic fractures.

Background and Purpose: Periprosthetic tibial plateau fractures are a rare but serious complication of UKA. Since they usually appear perioperatively they can be associated with sawing defects during implantation. The aim of the study was to evaluate fracture loads and fracture patterns under particular consideration whether extended vertical saw cuts reduce the stability of the tibial plateau and increase the risk of periprosthetic tibial plateau fractures.

Material and Methods: In 6 matched paired fresh frozen tibiae (donor data: f/m = 2/4, mean age 81.2 years, mean weight 61.7kg) tibial implantation of the cemented Oxford Uni was performed in group A and with an extended vertical saw cut of 10° in group B in a randomized fashion. Before fracturing the tibiae with a maximum load of 10.0kN under standard conditions, DEXA bone density measurement and standard X-Ray were accomplished. After load induction fracture patterns and maximum fracture loads were analyzed and correlated to BMD, BMI, bodyweight (BW), age and surface area of the tibial implant.

Results: In group A a maximum load of Fmax = 3.912 (2.346–8.500) kN lead to fractures, whereas in group B all tibiae fractured with a mean load of Fmax = 2.622 (1.085–5.036) kN. The difference was statistically different with p=0.028. The induced fractures were similar to those observed in clinical practice.

Between BMI and the maximum fracture loads inducing tibial plateau fractures a significant correlation could be proven for all tibiae (r=0.643).

Discussion: The observed fracture pattern showed metaphyseal fractures similar to those observed in clinical practise. Extended vertical saw cuts weaken the bone structure and therefore raise the risk of medial tibial plateau fractures. In our study extended vertical saw cuts of 10° reduce maximum fracture loads about 30%.

We recommend special training and modified instruments for inexperienced surgeons to minimize the incidence of extended vertical saw cuts and to reduce the risk of periprosthetic fractures.

Introduction: The medial unicompartmental knee prosthesis (UKA) is less invasive than total knee arthroplasty (TKA) and preserves undamaged structures of the joint. The range of movement and recovery are better in UKA, while postoperative pain reduction is at least equal to TKA. UKA have a higher revision rate than TKA (15% vs 10% after 10 years). One main reason for revision is mechanical loosening¹. There is a paucity of information regarding cement fixation of UKA. We compared jet lavage to conventional lavage with focus on cement pressures, interface temperatures and cement penetration.

Materials and Methods: UKA was performed in 10 paired entire human cadaver legs (Oxford Phase III, Biomet, Dordrecht, NL). Customized tibial implants and a pressure probe insert were used to measure the cement pressure anterior, posterior and near the implant fin during implantation and polymerisation. A drilling and fixation jig was used for standardized positioning of the three temperature probes. The polymerization heat was measured 5 mm below the bone surface at the medial and lateral plateau as well as under the fin. The same cementing technique was performed for all knees using Refobacin^® Bone Cement R. One side of the paired knees was cleaned using jet lavage, contra lateral cleaning was done with conventional lavage. The lavage volume was equal for both

Methods: AP radiographs were taken and digitalized to quantify the cement penetration areas and depths, using a pixel-analysis-software. Group comparisons were done with the Wilcoxon-Test using SPSS (SPSS Inc., Chicago, Illinois).

Results: Average cement pressure under the tibial implant is significantly higher for conventional lavage (avg cement pressure 25.69 ± 17.85 kPa, p= 0.005) than for jet lavage (avg cement pressure 13.28 ± 12.82 kPa). Mean temperature increase measured 5 mm below the bone surface medial and lateral, as well as under the implant fin, were statistically significant higher for the cementing technique with jet lavage (lat. 14.10 ± 5.72°C, p= 0.018/med. 8.49 ± 4.20°C, p= 0.176/fin 5.95 ± 1.92°C, p= 0.063) than for the conventional lavage (lat. 9.42 ± 5.17°C/med. 6.42 ± 2.21°C/fin 3.96 ± 2.03°C). On AP radiographs, cement penetration areas under the tibial implant were significantly higher for jet lavage (penetration area: 122.15 ± 33.94 sq mm, p= 0.046) than for conventional lavage (penetration area: 89.82 ± 23.92 sq mm).

Discussion: The use of jet lavage showed clear advantages in our cadaver studies. Jet lavage resulted in higher cement penetration despite of lower cement pressures under the tibial implant. The higher cement penetration lead to higher interface temperatures but exposure to high temperatures over 50 °C with a risk for bone necrosis could not be measured.

Radiolucent lines (RL) are a common radiographic finding following Oxford Uni knee arthroplastv. These RL are commonly seen at the bone-cement interface under the tibial tray and can only be evaluated using screened radiographs. These lines occur during the first year, are well defined and remain constant for years. The clinical relevance of this phenomenon is unclear. Pulse lavage has the potential to thoroughly clean the trabecular bone by clearing the bone from bone marrow and debris and can thereby facilitate cement penetration and interdigitation into trabecular bone.

Pulse lavage can reduce the occurrence of radiolucent lines under the tibial tray by improving cement penetration and interdigitation.

Since 2001 we routinely use pulse lavage before cementing the Oxford uni implants at the Orthopä-dische Universitätsklinik Heidelberg (group A). At Nuffield Orthopeadic Center, Oxford conventional irrigation has been used before cementing (group B). At a minimum follow-up of 1 year 56 screened AP radiographs of the knee after Oxford UCA have been blinded and evaluated for radiolucency and cement penetration by an independent observer. For standardized evaluation the cement bone interface has been devided into 4 zones and a specific algorithm for evaluation of the radiographs has been developed.

Complete radiolucencies were detected in 2 cases (4%) in group A and in 13 cases (23%) in group B (p=0,001). Partial radiolucent lines were seen in 32 knees of group A (57%) and in 40 knees (71%) in group B. In zone 1 RLs were found in 31 tibias (55%) in group A, in 32 tibias (57%) in group B. In zone 2 17 (30%) group A, 29 (52%) group B. In Zone 3 4 (7%) in Gruppe A, 20 (36%) in group B. In Zone 4 6 (11%) group A, 30 (54%) group B. The differences between group A and B were significant (P=0.001) in zones 2, 3 and 4.

In group A in 14 cases (25%) RL were limited to one zone, in group B in 5 cases (9%), respectively. In 12 cases (21%) 2 zones were affected in group A (12 cases (21%) group B). RLs in 3 zones were found in 4 cases (7%) in group A and in 10 cases (18%) in group B.

Mean cement penetration (mm) was 2,3mm in group A and in 1,4mm in group B. The use of pulsed lavage led to an increase in cement penetration by a factor of 1,6 (cement penetration in group A/B zone 1: 1,4mm/0,8mm; zone 2: 2,4mm/1,5mm; zone 3: 1,4mm/0,7mm; zone 4: 4,0mm/2,4mm).

The use of pulsed lavage significantly decreases the appearance of RLs at a minimum of 1 year follow-up by increasing cement penetration into cancellous bone. Even though the clinical relevance of tibial RLs in unclear we recommend the use of pulse lavage to improve cement penetration and interdigitation with cancellous bone. Unnecessary revisions due to misinterpretation of RLs may be prevented.