At arthroplasty the replaced knee is optimally reconstructed and aligned. The mechanical wash effect of pulsatile lavage clears the joint of bone, cartilage and cement particles generated during the operation. We analysed fluid obtained from the joint cavity after final saline lavage and retinacular closure. This fluid contains cement and non-cement particles that persist in the joint and likely contribute to early third body wear. These third body inclusions of cement happen early, much before the commonly described combination of PE and PMMA debris accumulates after wear. Non-cement particles like bone and cartilage were excluded.
In our results cement debris was seen across all patients in the study. However almost half the samples showed no cement particles suggesting that smaller particles at the nano scale are not visible under the light microscope. The generation of debris is inevitable at surgery but their residual numbers at closure should ideally be zero. Current surgical techniques are not able to achieve that despite impressive relief of pain and improvement of function in the medium to long term. The variable shape and surface geometry of primary cement debris suggests that it may get entrapped in the synovial lining and initiate an early synovial inflammatory response. Studies have demonstrated the number, size and shape of particulate debris to influence the inflammatory biological reaction that leads to periprosthetic osteolysis.7,8,9 Small particles in the range of 0.3–10 µm are more biologically active eliciting a marked inflammatory response. Some of these could be the cause of persistent synovitis in the early postoperative period and a cause of unexplained pain. Our study shows that smaller particles are present in larger numbers at closure. In the long term a sustained inflammatory cellular cascade at the bone-implant interface leads to loosening.10
Cement particles can function as a third body between the moving couple, get embedded in the PE insert leading to accelerated wear. Scratching and pitting from hard third body inclusions is seen in retrieved PE inserts of both fixed and mobile bearing articulations11. However these are joints that have failed over the medium to long term and the inclusions are a combination of PE wear and cement that is presumed to have migrated from the bone –implant interface. It is likely, as our study suggests, that some of these third body complexes are the result of an inability to render the joint free of hard, primary cement debris. Such debris can also scratch and affect the metal component early.
It is therefore essential that ways to decrease smaller particles that escape lavage are further investigated.
Pulsed lavage is widely believed to lower the risk of infection.12,13 Lavage also lowers free debris consisting of blood clots, cartilage, hard and soft bone and cement. Additional lavage does not dislodge all particles was shown by Yasuo Niki et al.13 In their in vitro study, volumetric particulate count became static after 9 liters of wash. Addition of antibiotics and other bactericidal agents such as betadine to saline aim to lower infection not debris. Addition of solvents to improve wash characteristics has not made progress.
Modifications in the method of cementation could decrease the number of free particles in the joint cavity. Incising the extruded cement sharply at the implant interface when doughy and removing it only after it has fully polymerised and hardened will prevent fragmentation and increase of cement debris. Preventing extrusion of cement beyond the edges of the implant by incorporating a disposable restrictor is an unexplored possibility.
We are aware of inaccuracies in our estimation of both number and size of primary cement debris. This is in part related to using an older method first described in 1964 for urinary WBC cytology to quickly answer a clinical question of suspected urinary infection. The decision to treat was based on whether WBCs in sufficient numbers were present in urine rather than their absolute count (G. R. F. Hilson et al)6. Similarly the method when applied here provides a glimpse of primary cement debris density of the joint at closure. New counting technologies such as infra-red spectroscopy can be used to improve accuracy but we had no access to such methods15. We have estimated the size of cement particles in relation to the diameter of nearby RBCs visible in the stains and not used formulae to estimate their volume. Instead, we averaged the number of particles per smear and used the previously described method to convert it to particles/cu.mm. Also, the samples were obtained after release of tourniquet and some of the debris could be trapped within blood clots rendering them inaccessible to the drain. This could have introduced an error of under counting. The advantage has been simplicity of the procedure such that both number and size of particles could be qualitatively estimated from H & E stains commonly available in histopathology departments.