Roughly one-third of patients who undergo a total knee arthroplasty (TKA) revision for periprosthetic joint infection (PJI) have knee instability one year after the procedure, according to new research. The findings were presented at the annual meeting of the American Academy of Orthopedic Surgeons (abstract 7208).

"The rate of knee instability in our study is unexpectedly high," said Jose Rodriguez, MD, study co-author and orthopedic surgeon at Hospital for Special Surgery. "Outcomes of patients undergoing a revision for PJI might not be as good as patients might have thought with longer term follow-up."

PJI is one of the most devastating and frequent complications that can occur after lower extremity joint arthroplasty, such as total knee replacement. The incidence of PJI varies by institution, from about .5 to 4 percent, and often requires revision surgery. Multiple procedures are sometimes necessary to eradicate infection and restore functionality, and even with successful eradication of infection, patients may not achieve optimal functional outcomes.

Dr. Rodriguez and colleagues launched their study after recognizing that there were patients who reported that their knees felt looser after they had a TKA revision for PJI. To shed more light on the issue, the researchers reviewed revision TKA cases for PJI involving four arthroplasty surgeons at a single institution in New York City from 2007-2017. Patients who had received a hinged prosthesis at initial reimplantation were excluded from the analysis. The researchers collected data on demographics, patient comorbidities (including diabetes, neuromuscular disorders, inflammatory arthritis, and hip or foot deformity), implant type, subjective reports or objective findings of instability, and need for further surgery.

There were 45 patients in the study, including 27 of whom had a minimum of 1 year of follow up. "Among the patients who had one-year follow-up, 33 percent had functional instability. Their knee moved more than what we would consider ideal," said Dr. Rodriguez.

The researchers also found that the risk for instability increased with a greater number of total surgeries performed on the ipsilateral knee. Varus-valgus constrained polyethylene was used in 70.4 percent of cases, but there was no significant correlation between using varus/valgus constraint and the presence or absence of instability. Constraint is defined as the effect of the elements of knee implant design that provides the stability needed to counteract forces about the knee after arthroplasty, in the presence of a deficient soft-tissue envelope. Determining the amount of constraint necessary can be challenging. Dr. Rodriguez said the study showed that a varus/valgus constraint insert alone may not be sufficient to prevent progressive instability, as the soft tissues stretch following treatment for infection.

"The degree of soft tissue tension that is achieved after treatment for periprosthetic infection at the time of surgery may not represent what it will be afterwards, because of the degree of swelling that exists when you are re-implanting following treatment for infection. After infection, the soft tissue remains swollen and may not give a suitable indication for the use of varus/valgus constraint at that time," said Dr. Rodriguez. "The subjective assessment of soft tissue tension at the time of re-implantation may not be an accurate predictor of stability at one year. We might need a lower threshold to use a hinged prosthesis or plan to stuff the joint tighter than what we might otherwise think ideal."

The study was a collaborative effort among researchers at HSS, Northwell Health/Lenox Hill Hospital, and Columbia University Medical Center. The researchers say that prospective studies with larger sample sizes are needed to shed light on when patients can benefit from increased constraint, such as a hinged prosthesis.