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Under the spotlight - The Unispacer - a minimally invasive alternative to knee replacement surgery

The UniSpacer, heralded as a minimally invasive alternative to knee replacement, has been in use for about five years now. The pioneers are beginning to present their experience to the world, amidst some controversy. I took time out to ask Dr David Trotter of the Center for Sports Orthopaedics, SC in Illinois (USA) a few questions -

KNEEguru: Dr Trotter, thank you for participating in this question and answer session. I know our readers are very keen to get answers to their questions about the UniSpacer.

Firstly, would you say I was right? - that the idea of the UniSpacer is that it slots into the gap of a missing meniscus and improves the forces through the knee in order to delay arthritic destruction consequent on previous total meniscectomy?

Dr Trotter: No. It's more than that. The UniSpacer procedure is relevant to patients with joint space narrowing on one side of the knee, where the joint cartilage ('articular cartilage') is becoming damaged by the altered forces through the knee. This is the case whether the meniscus has ever been acutely torn at all.

If the joint space narrowing is on the medial (inner) side the bowing of the leg is called 'varus' deformity, and the primarily affected region is termed the 'medial compartment' of the knee. The articular cartilage of the medial compartment frequently is either thinner or just softer than normal, especially in the more active, younger end of the spectrum. This often occurs even with an intact meniscus (or rim).

Overall, the key issue is the narrowing of the medial joint space on a standing film. Joint space narrowing often results in increasing pain due to the dysfunctioning cartilage support or 'bone on bone' effect. This can lead to instability due to increased joint compression (like a hammer in dish effect) in the stance phase in that compartment.

KNEEguru: You talk about 'standing' films. What do you mean?

Dr Trotter: These are X-rays of the knee taken while the patient is standing and bearing their body weight. Varus is sometimes difficult to always pickup on clinical examination. In fact, some people's knees look almost valgus (knock-kneed) but when you get standing films, you can see medial joint space narrowing. Of course, we must always ensure that the primary problem is from the medial compartment.

KNEEguru: So, summarise that again for us, please.

Dr Trotter: Sure - "The 'spacer' is optimal for that symptomatic patient who has failed reasonable non-operative treatment and who has a tendency for slight (or sometimes dramatic) medial joint space narrowing on x-rays."

We look especially closely at the medial joint space in standing films. If the patient points to the anterior medial (inner front) knee (although often they can't quite localize the pain site quite that well) and, if the medial joint space is decreased as compared to the lateral (outer) space, then they are quite likely (if the ligaments are intact or repaired) a good candidate for symptom relief from a UniSpacer.

KNEEguru: Can you tell us when the first UniSpacer procedure was performed, and how many patients have had this procedure so far?

Dr Trotter: The first UniSpacer was performed in May, 2000 and there have been thousands performed globally, including in both knees of a sizable number of patients.

KNEEguru: But it is still early days and the medical literature will still be sparse. How long in your opinion does a new operation like this take to fully evaluate once surgeons start using it?

Dr Trotter: The best' medical studies are typically those that have a large enough number of patients that have been clinically followed for a long enough period so that a trend or pattern (or lack thereof) appears to have emerged. The standards of just what constitutes 'large enough' and 'long enough' can vary depending upon the branch of medicine and/or form of treatment being assessed.

However, remember that the UniSpacer is a form of 'interpositional arthroplasty' (a joint replacement between the femur and tibia), and the concept of interpositional arthroplasty predates the early versions of the total knee replacement procedures. Therefore, our understanding of many of the issues goes back well before May 2000.

KNEEguru: Can you tell us more about that.

Dr Trotter: Certainly. In the 50's and 60's, many patients were treated with metallic tibial hemiarthroplasty, including implants developed by MacIntosh and McKeever. These implants required relatively minimal upper tibial (lower half of the joint space) bone resection and occasionally required the removal of minimal femoral condyle (upper half of joint) to allow for proper function.

MacIntosh and Hunter reported, in 1972 JBJS [Journal of Bone and Joint Surgery], an average follow-up of 3.5 years, 135 implants. They reported that 80% of the patients with OA (osteoarthritis) had good results.

Three authors have reported on their experiences with McKeever Hemiarthroplasties in OA patients. The studies, published in 1972, 1985 and 1985 JBJS reported as good-excellent results in the 3 studies:

  1. 89% (3 year average follow up)
  2. 70% (8 yr. av. f/u) and 79% in pts. under 64, and
  3. 75% (5 yr. av. f/u)

Overall however, as compared with partial (two-piece unicompartmental) or with total knee replacement, available data on the UniSpacer procedure is significantly less.

Despite that, the relatively early results of the refined UniSpacer procedure (including, in my experience, the combination of UniSpacer with removal of a large portion of the synovial tissue that contributes to the production of excessive arthritic fluid/inflammation) are promising.

KNEEguru: In June 2003 a Dr. Friedman presented a paper quoting early reports of 8-10% dislocation, and that he himself had to revise 23% of his own Unispacer cases. Have things moved on? Were the earliest users using a different technique to your current technique?

Dr Trotter: Absolutely. The rate of dislocation has plummeted to between 1-3% in the hands of experienced users of the refined/modern surgical technique. The overall revision rate has likewise been dramatically reduced to a similar infrequent range, again in the hands of those specially trained surgeons that have learned from the education and experience of the earliest pioneers.

Today, not only have we benefited from a far better experiential base regarding UniSpacer implant sizing; we also have benefited from an improved surgical technique that addresses issues such as the fluid producing knee tissue known as the synovium. At the Center for Sports Orthopaedics, SC, we have specifically added an extra process in the UniSpacer technique in order to remove a significant portion of this tissue that can be associated with inflammation in patients with osteoarthritis. Early results of this UniSpacer-synovial ablation combination appear quite promising.

KNEEguru: I understand that patient selection has been shown to be very important in this procedure. What are your own selection criteria?

Dr Trotter: To date, we have performed the UniSpacer technique in patients ranging from a 33 year old pediatric nurse to a still active 88 year old (who recently requested and now has a UniSpacer in each knee.) At the Center for Sports Orthopaedics, UniSpacer candidacy is determined not by age but by the anatomy of a patient's arthritic knee.

Appropriate candidates for the procedure include patients that:

  • have moderate to severe knee pain associated with activities. The pain is often (but not always) localizable to the inner aspect of the knee. "It hurts here doc" is a common refrain heard in our center.
  • We routinely obtain standing knee x-rays to determine if the patient has a narrowed medial (inner knee) joint space upon weight-bearing. During the evaluation, if the patient's medical history, exam findings and x-rays all support that the overwhelming majority of pain is coming from the medial compartment of the knee joint, then the patient would be a candidate.
  • When a patient has not had adequate relief with oral medications or injectibles and/or bracing, then he or she would be a potential candidate.
  • It's important to emphasize that a patient's ACL (anterior cruciate ligament) must be intact (either never injured or repaired).
  • In the operating room, we also surgically address certain patello-femoral joint pain with a release of the lateral retinacular tissue adjacent to the kneecap. This tends to both realign and untether the patella from the same chronic pressure producing region. Lateral compartment disease (if applicable) can be also addressed during the initial arthroscopic phase of the UniSpacer procedure.

Having the largest series of UniSpacer patients in the Midwest region of the United States, we are continually cognizant of optimizing patient selection. Also, postoperative therapy techniques are important and include cooling of the knee, minimal to no bracing, and, the use of a CPM (continuous passive motion) device for optimizing knee motion.

KNEEguru: So my initial assumptions were clearly quite wrong. The Unispacer has a broad application and is not just for patients who have had a medial meniscectomy?

Dr Trotter: Yes. The UniSpacer implant is an alternative to total and/or partial knee replacement surgery. An affected patient's knee has either developed chronic degeneration of the inner knee compartment through natural 'wear and tear', or the condition may have developed after a knee injury that resulted in a cartilage or meniscal injury. Regardless of whether or not a patient has ever undergone any prior surgery to attempt to address any known prior cartilage problem, the UniSpacer is designed to relieve pain and provide stability due to malfunctioning degenerative or absent cartilage.

The UniSpacer has been currently designed for the (medial) inner side of the knee only as the frequency and degree of medial compartment deterioration far exceeds that of lateral or outer knee compartment cartilage degeneration.

KNEEguru: What is the difference in outcome between a Unispacer procedure and an osteotomy?

Dr Trotter: In my opinion, the outcomes of UniSpacer can be superior to an osteotomy (in the hands of UniSpacer surgeons who are routinely implanting the device) for these reasons:

  • Osteotomy procedures (surgical bony realignment) require anatomy altering bone cuts that can sometimes affect future joint replacement procedures.
  • Osteotomy typically requires extensive initial postoperative immobilization and decreased weight bearing not involved post UniSpacer.
  • Regarding osteotomy, a Dr. Coventry (JBJS 1993) noted that patients who had undergone a high tibial osteotomy and whose body weight exceeded approximately 1.3x ideal had overall poorer results than those that had more ideal body weights. Weight does not appear to be an important factor with the Unispacer.
  • Also, Coventry had reported some postoperative motion loss that I have not noted in the current UniSpacer series of patients.
  • In some series, a degree of recurrent bowing after a relatively limited number of years has been noted post osteotomy.

Overall, it is difficult to directly compare UniSpacer results with osteotomy. The results of osteotomy are longer term, have been not unreasonable, and have been typically reported in terms of maintenance of the joint realignment. UniSpacer and/or joint replacement series have been reported in terms of scoring scales discussing pre- and post-operative pain relief and functional capabilities.

In the hands of UniSpacer surgeons who are routinely implanting the device where indicated for osteoarthritis, the majority of results have revealed approximately 80-100% pain relief. (These percentages are derived subjectively from UniSpacer patients who are asked about the magnitude and frequency of pain reduction, the increased resolution rate of pain (if it occurs), and from accepted scales of pre-and post-op functional abilities.

KNEEguru: You mentioned 'unicompartmental' knee replacement (arthroplasty). What are the differences between a UniSpacer and a unicompartmental?

Dr Trotter: Of the medial, lateral and patello-femoral compartments of the knee, it is widely accepted that the condition of symptomatic osteoarthrosis ('arthritis') typically affects the medial (inner) compartment much more frequently and/or with a greater severity than the lateral compartment.

The term Uni-knee has become synonymous with a number of implants or 'constructs' (from a number of implant companies) designed to surgically treat the medial (inner) or lateral (outer) compartments of the arthritic knee. It has also been referred to as a unicompartmental knee implant, as it is only placed in the compartment (typically medial) that is felt to contribute to the overwhelming majority of a patient's symptoms of arthritic pain and/or instability.

The construct typically involves the insertion of a metallic component on the lower end of the femur that articulates with a plastic (or plastic on metal) component on the upper tibia. These components (in order to be inserted) do require bone cuts/resection, and also fixation with bone cement. The construct is designed to act as a low friction partial knee replacement. If future revision to a bi- or tri-compartmental (total knee replacement) is necessary, removal of the two-piece components and cement are necessary.

More recently, the term Uni-knee has broadened to include the UniSpacer implant. The UniSpacer device, unlike the two-piece construct, is a single component alloy of metallic Cobalt and Chromium, designed for durability and wear characteristics. The UniSpacer does not require significant bone cuts/resection/ligament work or cement fixation, all of which can be inherent in a two-piece metal on plastic unicompartmental construct.

The UniSpacer acts as an interpositional 'spacer', in effect taking the place of the worn/degenerated knee cartilage. The early positive clinical outcomes of pain reduction and knee function restoration have been attributed to the redistribution of weight bearing loads and to restored knee stability.

KNEEguru: Are there any contraindications for the procedure?

Dr Trotter: Contraindications against being a candidate would be either if a patient's outer (lateral) compartment is essentially involved as much (or more severely) than the inner (medial) compartment. The UniSpacer is currently not designed for lateral compartment disease. Also, severe patellofemoral compartment may be a challenge. However it would not necessarily preclude a patient's candidacy from UniSpacer, as it can be addressed surgically.

FOOTNOTE from Dr Trotter: There have been some cases reported in which second look scopes (rarely needed) have revealed some apparent new cartilage formation (of unknown quality). This is speculative but may account for the perceived improvement of some patients up to one year post-op.

Related Links:

Dressler K, Ellerman A. Unispacer - a new minimally-invasive therapeutic concept for the isolated medial knee joint disease

Friedman MJ. Unispacer

Hallock RH, Fell, BM. Unicompartmental tibial hemiarthroplasty: early results of the Unispacer knee.

Scott RD. UniSpacer: insufficient data to support its widespread use.

Geier KA. The UniSpacer for knee osteoarthritis.

Hallock RH. The UniSpacer Knee System: have we been there before?

Updated: 17 Apr, 2013

Dr Sheila Strover

Clinical Editor
BSc (Hons)

Dr Sheila Strover is the founder of the KNEEguru website. Although not a knee surgeon, she has a sound understanding of knee surgery and...

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