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No Factor Predicts Need for Ultracongruent Insert During TKA

Key findings

  • This study reviewed 117 patients who underwent primary unilateral total knee arthroplasty (TKA), 39 using an ultracongruent insert and 78 patients using a standard cruciate-retaining (CR) implant, looking for preop factors that predict implant choice
  • Preoperative demographic, comorbidity and radiographic data were similar between the two groups
  • Range of motion and radiographic alignment were also similar postoperatively
  • The proportion of patients for whom tibial slope increased postoperatively was greater for the UC group compared to the CR group (p=0.018)

One of the most enduring debates in orthopedics is how to manage the posterior cruciate ligament (PCL) during primary total knee arthroplasty (TKA). The multiple implants now available include cruciate-retaining (CR), cruciate-sacrificing and posterior stabilizing (PS), high-flexion CR and high-flexion PS bearing surfaces and fixed-bearing ultracongruent polyethylene (UC) inserts.

UC components are designed to improve on PS implants by increasing knee stability and balancing while avoiding the risks of additional bone resection and cam–post dislocation. However, multiple studies comparing UC inserts with standard PS or CR implants have not pinpointed any differences in radiologic parameters or outcomes. Because of this, the decision to use a UC insert instead of a CR implant had usually been left to the surgeon's discretion, based on intraoperative assessment of the PCL.

Then, a retrospective study published in Knee Surgery, Sports Traumatology, Arthroscopy showed that increased preoperative angle of flexion contracture, increased preoperative posterior tibial slope and reduced femoral component size predicted the need for intraoperative conversion from a CR to a standard PS design.

Orthopedic surgeon's Hany S. Bedair, MD, and Christopher M. Melnic, MD, and surgical research fellow Akhil Katakam, MD, MBA, all from the Center for Hip and Knee Replacement at Massachusetts General Hospital, and colleagues expanded this line of inquiry by examining variables that may predict the need for conversion from CR to a UC component. However, as they report in Surgical Technology International, they found no preoperative patient or surgical factor that does so.

Study Methods

This study involved patients of a single surgeon who used either the CR or UC components of the Zimmer Persona Total Knee System. The researchers identified 39 patients who underwent primary unilateral TKA using the UC bearing between 2013 and 2018. Those patients were compared with 78 patients who received a standard CR bearing during the same time period.

Preoperative Factors

The UC and CR groups were similar in:

  • Age
  • Sex
  • Body mass index
  • The comorbidities studied (hypertension, cardiac disease, lung disease, diabetes, current treatment for depression or anxiety and never-smoking)
  • Radiographic measures of posterior tibial slope angle (PSA), condylar offset and degree of varus deformity

Intraoperative Factors

  • Tourniquet times did not differ between the UC and CR groups
  • Similar proportions of the two groups received spinal anesthesia
  • No implant variable varied between groups, including the size of the polyethylene component, femoral component and patellar component

Postoperative Factors

The groups were similar with respect to:

  • Complication rates
  • Postoperative knee flexion and extension range of motion
  • Radiographic measures of PSA, condylar offset and degree of varus deformity

There were no reoperations in the CR group, but four in the UC group (P=0.01); reasons included range of motion deficits, flexion instability and poor clinical outcome. The pre- to postoperative change in PSA and condylar offset did not differ between the groups. The tibial slope increased in 44% of patients in the UC group compared with 22% in the CR group (P=0.02).


The latter finding is surprising because the UC implant in this Zimmer system was designed to reduce tibial slope by 2º compared with the CR implant. Having increased slope during the surgery may lead to more difficulty balancing the flexion gap, requiring the use of a UC component. However, no preoperative clinical or radiographic factor was found to be predictive.

Learn more about the Center for Hip and Knee Replacement at Mass General

Refer a patient to the Department of Orthopaedics


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