Heterotopic ossification after total knee arthroplasty.

Atamaz F, Aydogdu S, Hepguler S, Sur H. Acta Orthop Traumatol Turc 2006;40(3):202-206.

This is the editor's interpretation of a paper published in the orthopaedic literature in 2006 - our attempt to make relevant medical articles accessible to lay readers. If you wish to read the original it is easy to ask your librarian to obtain a reprint for you from any medical library.

The authors of this paper were interested in the incidence of hereterotopic ossification (HO) in their own patients after total knee replacement. [Ed note: Heterotopic ossification is a condition where bone becomes stimulated to form abnormally in the soft tissues.]

They looked retrospectively at the X-rays of 226 consecutive patients of theirs who had had a total knee replacement, and who had been followed up at 6 weeks, and then at 3, 6 and 12 months after their surgery. They were interested in comparing pre-op X-rays and then these post-op X-rays to determine the incidence in their own practice and at what stage HO might be picked up on X-ray should this complication happen.

The paper begins with a review of the literature up until that time. The authors point out that although HO is quite common after hip replacements, it is much more uncommon after knee replacements.  According to the literature, when it does occur it usually makes an appearance on X-ray at about 6 weeks (range 3-8 weeks) after surgery, accompanied by signs of local redness, warmth and swelling of the knee and symptoms of pain and fever. If the signs are present on X-ray at 6 weeks, then from this time the condition should be apparent in all subsequent X-rays.

The condition was first reported consequent to total knee replacement in 1973, and since then - as surgeons became more aware of the condition and started monitoring their own patients more carefully - it was found that the incidence is higher than was initially thought. Reviews in 1986 and 1993 reported as many as 47% of patients having this complication after knee replacement! However, in this study of their own patients, the authors found evidence of HO only in two knees (0.9%) at the sixth month evaluation of lateral X-rays. It was grade 1 in both knees and localized at the above the supracondylar area (ie the rounded ends) of the distal femur (ie the lower part of the thighbone).

This is the grading classification they were following (Figgie classification -

  • Grade 0 No evidence of HO
  • Grade I Progressive HO in at least 1 compartment of the knee, without spur formation.
  • Grade II Progressive HO in at least 1 compartment of the knee, with spur formation (> 1mm)
  • Grade III Complete bony ankylosis.

They were surprised that there was such a low incidence of HO in their own patient group, and concluded that it might be due to the fact that they do not use any blood-thinning agents for their patients undergoing knee replacement. This was only speculative.

Their own review of the literature identified factors that others thought might be important in stimulating the onset of HO -

Neither of their patients had had any of these, except that they both had osteoarthritis.