High-flexion knees — too much flex or just right — it depends on the patient

High flexibility may be unnecessary in some patients, but early follow-up with new designs finds good function and knee scores.

Issue: March 2006

ORLANDO, Fla. — Whether or not to use high-flexion knee designs for total knee arthroplasty largely depends on preop flexion abilities, say two experts who debated the issue at the 22nd Annual Current Concepts in Joint Replacement Winter 2005 Meeting.

Ultimately, high-flexion designs may be unnecessary in the United States because the typical need for postoperative flexion is no more than 120°, according to Richard D. Scott, MD, of Harvard Medical School in Boston. “We should not encourage high-flexion knee arthroplasties unless they are unicompartmental replacements,” he said, adding that “120° to 130° of flexion is perfectly adequate for knee arthroplasty patients and will serve them well and safely.”

High flexion in other countries

On the other hand, Michael A. Kelly, MD, of Insall Scott Kelly in New York, said high flexion is necessary for patients in Middle and Far Eastern countries — where higher flexion is common and necessary for everyday activities. What’s more, Kelly said, the indications for younger U.S. patients are increasing, and those with good preoperative range of motion want to maintain or improve that range of motion after knee replacement.

“Obviously [in] the patient with good preoperative flexibility, good postoperative management of his pain, rehab and motivation and the knee itself, the surgeon must create a flexion-friendly environment, removing the posterior condylar impingement issues and creating a balanced knee to achieve optimum flexion,” Kelly said.

120° is enough

Scott explained that the average person needs 70° flexion to walk on level ground, 90° to ascend most stairs, 100° to descend most stairs, about 105° to stand up from a regular chair and about 115° to stand up from a low sofa. Therefore, “the vast majority of patients achieving 120° of flexion are very satisfied,” he said.

Ultra-high flexion also brings disadvantages: “Current designs require increased bone resection, potentially there could be adverse effects on poly wear or component fixation and potentially dangerous high patellofemoral forces,” Scott said, noting also that “Increased posterior condylar resection ... might be considered a significant price to pay in your younger patients.”

A recent study also suggests that patients who achieve high flexion do not use it in everyday activities. In fact, patients with the potential for 150° of flexion, only used up to 120°. “That implies the reason for these desig ns is not to achieve the high flexion, but to protect those patients who actually achieve that amount of range of motion,” Scott said.

Rollback and posterior wear

In his experience, Scott said, some patients who achieved ultra-high flexion experienced excessive rollback and late posterior wear. To protect against the posterior wear, current high-flexion designs increased contact areas, but researchers found that the patella reacts poorly to such high forces.

In early total condylar experiences, patients who achieved 85° to 90° of flexion experienced few patellar stress fractures, even though the implants contained large central fixation lugs, Scott said. However, when patients started to achieve 110° to 115° of flexion, stress fractures increased, while components with smaller central lugs had fewer stress fractures but a higher loosening incidence.

“The current high flexion designs ... don’t protect the patellofemoral joint from these high forces,” Scott said.

Additional bone resection

Kelly agreed that high-flexion designs require additional bone resection — approximately 2 mm. However, in one case, additional bone resection permitted increased conformity and safe deep flexion of 155° without edge loading, he said.

At Kelly’s institution, surgeons conducted a study using high-flexion knee designs on a group of active patients, all with osteoarthritis and all under 55 years old, finding a 94% survivorship at 18 years. At 21-year follow-up, researchers found only a slight drop in the Tegner score and knee scores.

Flexion-friendly environment

Several designs introduced to the market in the past five years allow surgeons to create a “flexion-friendly environment,” Kelly said, rather than a prosthesis itself achieving more flexion. Such environments reduce the risk of edge loading and prevents the posterior condyles from digging in to the polyethelene.

The shared design features include posterior condylar offset with improved deformity, deep flexion, improved contact area and a recess in the anterior polyethelene to accommodate increased excursion in the patellar tendon. “If we look at some of these features a little more, on an international basis, many include the addition of the mobile bearing to accommodate the increased internal rotation in maximum flexion,” Kelly said.

No osteolysis, loosening

In addition, to address the issues of increased patellofemoral forces and patellar tendon excursion, many designs aim to recess the anterior aspect of the knee to avoid impingement and allow the patellar tendon ample excursion.

Using these new designs, Kelly’s institution conducted an early follow-up study on 58 knees in 47 patients. Radiographically, they found no osteolysis, loosening or wear issues, as well as excellent function and knee scores. “These [patients] were probably preselected ... with excellent preoperative range of motion,” Kelly said.

For more information:
Scott R. High-flexion knee designs: More hype than help — Affirms. #96.
Kelly M. High-flexion knee designs: More hype than help — Opposes. #97. Presented at 22nd Annual Current Concepts in Joint Replacement Winter 2005 Meeting. Dec. 14-17, 2005. Orlando, Fla.