Medical Policy
Subject: Unicondylar Interpositional Spacer
Document #: SURG.00053Publish Date: 04/07/2021
Status: ReviewedLast Review Date: 02/11/2021

This document addresses unicondylar interpositional spacers, which have been proposed as a treatment for early-stage osteoarthritis (OA) of the knee. The spacers are kidney-shaped metal inserts which are designed to function as a self-centering bearing in the knee. The spacer fits between the natural bone structures of the knee and is not affixed with cement or screws.

Note: Please see the following related document for additional information:

Position Statement

Investigational and Not Medically Necessary:

Use of unicondylar interpositional spacers is considered investigational and not medically necessary for all indications.


Currently, there are few studies published in the medical literature that allow for adequate evaluation of the use of unicondylar interpositional spacers in the clinical setting. One case series conducted by Hallock and colleagues (2003), reported 1- and 2-year data on 71 UniSpacer® Knee System implants in 67 individuals as a treatment of isolated medial compartment OA of the knee. The mean age and weight of these individuals was 54 years and 94 kg (207 pounds), respectively. All knees were evaluated using the Lysholm scoring scale, the Knee Society clinical rating system, radiographic limb alignment, and range of motion (ROM). The mean Lysholm score improved 88% in the 1-year group and 140% in the 2-year group. The mean Knee Society knee score improved 169% and 193%, respectively. The mean Knee Society function score improved 31% and 65%, respectively. Five implants (7%) were revised to total knee arthroplasty (TKA) and 10 implants (14%) were revised to another UniSpacer Knee System implant. The researchers concluded that the early results suggest that an intra-articular metallic spacer is a viable treatment option for OA in younger individuals.

Sisto (2005) reported on the experience of a single surgeon with the UniSpacer as a treatment for isolated medial compartment arthritis of the knee. From April 2002 through November 2002, 37 UniSpacer arthroplasties were performed in 34 individuals (18 women [19 knees] and 16 men [18 knees]) who had a mean age of 55 years. The Ahlback radiographic evaluation scale was used to grade the severity of arthritis. Twelve individuals had a previous arthroscopic meniscectomy. The mean preoperative Knee Society function score was 60 points (range, 40 to 80 points), and the mean preoperative Knee Society objective score was 62 points (range, 40 to 76 points). After a mean duration of follow-up of 26 months (ranging from 24 to 29 months), the mean postoperative total function score was 69 points (range, 40 to 82 points), and the mean Knee Society objective score was 72 points (range, 45 to 88 points). There were 10 good, 15 fair, 12 poor results, and no excellent results. Six of the 12 poor results were in knees that had dislocation of the UniSpacer that then resulted in the individuals undergoing a total knee arthroplasty. The researchers concluded that the UniSpacer arthroplasty should not be used as a treatment of degenerative arthritis of the medial compartment of the knee.

Bailie and colleagues (2008) conducted a prospective study to determine the early clinical outcomes of 18 consecutive individuals with OA of the knee that were treated with the UniSpacer knee implant between June 2003 and August 2004. Between 3 and 6 months after surgery, 17 individuals (94%) reported persistent symptoms. At an average 17 month follow-up (3 to 26 months range), 12 (67%) individuals required further interventions, and 8 (44%) required revision within 2 years. The authors concluded that the use of the UniSpacer in isolated medial compartment OA is associated with a high rate of revision surgery and provides unpredictable pain relief.

The American Academy of Orthopaedic Surgeons (AAOS) 2013 evidence based guideline on treatment of osteoarthritis (OA) of the knee includes a consensus recommendation that free-floating (un-fixed) interpositional devices not be used in individuals with symptomatic medial compartment OA. The work group noted that there is a lack of evidence supporting this device.

In light of the lack of well-designed studies supporting the efficacy of the device, conclusions regarding the net benefit of unicondylar interpositional spacers cannot be drawn. Further studies on outcomes, particularly in comparison with standard treatments, are essential to determine efficacy.


Description of Disease

Osteoarthritis is a degenerative condition of the joints and is the most common form of arthritis. OA commonly affects hands and weight-bearing joints such as the knees, hips, feet, and spine. The condition is characterized by a progressive breakdown of the cartilage lining the articular ends of bones required for proper cushioning and smooth function of joints. The breakdown of this cartilage leads to bones rubbing against each other causing pain, loss of movement, and destruction of the joint. OA affects an estimated 20.7 million Americans, mostly after age 45, with women more commonly affected than men. The severity of OA varies widely. Most people with OA report some limitation in movement or daily activities due to pain or diminished range of motion.

There is no cure for OA. Treatment focus is on reducing pain and improving joint movement through exercises to keep joints flexible and improve muscle strength, medications and heat/cold therapy to control pain, joint protection to prevent strain or stress on painful joints, weight control, and surgery.

Description of Technology

Many different types of surgical procedures have been developed to treat OA of the knees including knee debridement, high tibial osteotomy, and partial (unicompartmental) and total knee arthroplasty (replacement). Although often beneficial, such procedures can also be associated with long recovery periods, compromise of the joint to future interventions, and treatment failure after a short period of improvement. In addition, most procedures do not address mechanical alignment issues.

The unicondylar interpositional spacer has been proposed as an alternative to the more invasive surgical interventions mentioned above. This device is used in individuals with joint degeneration/destruction limited to one compartment (normally the inside or medial compartment) of the knee joint. The unicondylar interpositional spacer consists of a metallic hemispherical spacer that can be surgically implanted in the joint space between the affected bone ends. The device is self-aligning and requires no cement or screws to keep it in place.

Proposed Benefits

The implantation of unicondylar interpositional spacers has been proposed as a less invasive surgical procedure for selected cases of OA of the knee. One suggested advantage is that the bone ends rub on the polished metal surface of the device sparing further bone wear from bone to bone contact. In addition, it has been suggested that the less invasive surgical procedure may decrease further joint destruction common in many other knee operations.

Possible Risks

Implanting a unicondylar interpositional spacer carries the risks of any significant surgical procedure including infection and bleeding into the site. In addition, the surgical treatment may eventually fail necessitating conversion to a full unicompartmental or total knee arthroplasty. In a review of the Unispacer, Scott concluded the eventual role of the Unispacer is uncertain and the procedure is technically demanding, making its widespread success unlikely (Scott, 2003).


Unicompartmental: Related to either the inside (medial) or outside (lateral) half of the knee joint.

Unicondylar interpositional spacer: A specialized hemispheric metallic device that can be surgically implanted into the joint space of the knee; this device has been used as a treatment for arthritis that affects only part of the knee (unicompartmental arthritis).


The following codes for treatments and procedures applicable to this document are included below for informational purposes. Inclusion or exclusion of a procedure, diagnosis or device code(s) does not constitute or imply member coverage or provider reimbursement policy. Please refer to the member's contract benefits in effect at the time of service to determine coverage or non-coverage of these services as it applies to an individual member.

When Services are Investigational and Not Medically Necessary:
When the code describes a procedure indicated in the Position Statement section as investigational and not medically necessary.




Unlisted procedure, femur or knee [when specified as implantation of unicondylar interpositional spacer in the knee]



ICD-10 Procedure



Insertion of spacer into right knee joint, open approach


Insertion of spacer into right knee joint, percutaneous approach


Insertion of spacer into right knee joint, percutaneous endoscopic approach


Insertion of spacer into left knee joint, open approach


Insertion of spacer into left knee joint, percutaneous approach


Insertion of spacer into left knee joint, percutaneous endoscopic approach



ICD-10 Diagnosis



All diagnoses


Peer Reviewed Publications:

  1. Bailie AG, Lewis PL, Brumby SA, et al. The Unispacer knee implant: Early clinical results. J Bone Joint Surg Br. 2008; 90(4):446-450.
  2. Emerson RH, Potter T. The use of the McKeever metallic hemiarthroplasty for unicompartmental arthritis. J Bone Joint Surg Am 1985; 67(2):208-212.
  3. Hallock RH, Fell BM. Unicompartmental tibial hemiarthroplasty: early results of the Unispacer® knee. Clinical Orthopedics and Related Research. 2003; 416:154-163.
  4. Scott RD. The UniSpacer®: insufficient data to support its widespread use. Clinical Orthopedics and Related Research. 2003; 416:164-166.
  5. Sisto DJ. Unispacer arthroplasty of the knee. J Bone Joint Surg Am. 2005; 87(8):1706-1711.

Government Agency, Medical Society, and Other Authoritative Publications:

  1. American Academy of Orthopaedic Surgeons (AAOS). Treatment of Osteoarthritis of the Knee 2nd Edition. May 18, 2013. Available at: Accessed on January 19, 2021.
  2. U.S. Food and Drug Administration (FDA). 510(k) Summary:
Websites for Additional Information
  1. Arthritis Foundation. Osteoarthritis: Available at: Accessed on January 19, 2021.
  2. National Institute of Arthritis and Musculoskeletal and Skin Diseases. Handout on Health: Osteoarthritis: Last Reviewed: October 2019. Available at: Accessed on January 19, 2021.

Interpositional Spacer
Knee Interpositional Mini-Repair System
Preservation Unicondylar Knee Prosthesis
Unicompartmental Knee Arthroplasty
Unicompartmental Osteoarthritis
Unicondular Interpositional Spacer
Unicondylar Interpositional Spacer

The use of specific product names is illustrative only. It is not intended to be a recommendation of one product over another, and is not intended to represent a complete listing of all products available.

Document History






Medical Policy & Technology Assessment Committee (MPTAC) review. Updated Rationale, References, Index, and Websites for Additional Information sections.



MPTAC review. Updated Rationale and Websites for Additional Information sections.



MPTAC review. Updated Background and References sections.



MPTAC review. Updated Rationale, References and Websites for Additional Information sections. The document header wording updated from “Current Effective Date” to “Publish Date.”



MPTAC review. Updated review date, Rationale, References and History sections.



MPTAC review. Updated review date, References and History sections. Removed ICD-9 codes from Coding section.



MPTAC review. Updated review date, References and History sections.



MPTAC review. Updated review date, References and History sections.



MPTAC review. Updated review date, References and History sections.



MPTAC review. Updated review date, Rationale, References, and History sections.



MPTAC review. Updated review date, References and History sections.



MPTAC review. Updated review date, Background/Overview, References and History sections.



MPTAC review. Removed brand names from the description and position statement sections of the document. Updated review date, rationale, references, index and history sections.



MPTAC review. Updated review date, references and history sections.



The phrase "investigational/not medically necessary" was clarified to read "investigational and not medically necessary." This change was approved at the November 29, 2007 MPTAC meeting.



MPTAC review. References, index and history sections updated. No change to position.



MPTAC review. References updated. No change to position.



MPTAC review. Revision based on Pre-merger Anthem and Pre-merger WellPoint Harmonization.

Pre-Merger Organizations

Last Review Date

Document Number



Anthem, Inc.



Unicondylar Interpositional Spacer

WellPoint Health Networks, Inc.




Unicondylar Interpositional Spacer as a Treatment of Unicompartmental Arthritis of the Knee


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