Implantable Shock Absorber Provides Superior Pain Relief and Functional Improvement Compared With High Tibial Osteotomy in Patients with Mild-to-Moderate Medial Knee Osteoarthritis: A 2-Year Report

Abstract

Objective: Up to 10 million Americans below the age of 65 years have symptomatic knee osteoarthritis (OA) and may not yet be candidates for arthroplasty. In response, a subcutaneous implantable shock absorber (ISA) that unloads the knee has been developed. The safety and effectiveness of ISA treatment were compared against a surgical unloading control, high tibial osteotomy (HTO).

Design: This was a prospective open-label cohort study with a historical control arm. Subjects underwent ISA placement or HTO. The primary endpoint was a composite variable combining pain, function, specific adverse events, integrity of implant or hardware, and conversion to subsequent surgery. Pain and function outcomes (Western Ontario and McMaster Universities Arthritis Index scores) were assessed through 24 months. Adverse events were tracked.

Results: The primary endpoint demonstrated superiority of the ISA arm versus the HTO arm, with 85.6% of ISA subjects meeting all criteria compared with 65.5% of HTO subjects. In addition, all 5 secondary endpoints showed superiority of ISA over HTO. At 24 months, the proportions of subjects considered responders were 95.8% (ISA) versus 87.9% (HTO) for pain and 91.7% (ISA) versus 81.3% (HTO) for function. The ISA procedure was well tolerated, with 13.4 days to full weightbearing status versus 58.0 days for the HTO arm.

Conclusions: Treatment with an ISA demonstrated noninferiority and superiority versus treatment with HTO in subjects aged 25-65 years who had OA of the medial knee. Treatment with ISA has high clinical benefit and is durable through at least 24 months.

Trial registration: ClinicalTrials.gov NCT03671213 NCT03838978.

Keywords: high tibial osteotomy; knee; osteoarthritis; shock absorber; unloading.

Figure 1.

The assembled knee shock absorber (top) and its placement when implanted (bottom).

Figure 2.

Subject disposition for both arms.

Figure 3.

Days to full weightbearing in the ISA and HTO arms. Boxplot indicates median (notch) and interquartile range (IQR), with whiskers identifying the highest and lowest data point within 1.5x IQR, and * indicates a statistically significant difference between the arms. ISA = implantable shock absorber; HTO = high tibial osteotomy.

Figure 4.

(A) WOMAC pain ratings decreased in both arms after treatment. Pain scores in the ISA arm (green) were significantly lower than those in the HTO arm (blue) at 6 weeks and 3, 6, and 24 months. (B) The percentage of pain improvement at each of the follow-up time points followed a similar trajectory. (C) The proportion of responders (at least 20% improvement from baseline and a change of ≥10 points) was significantly higher in the ISA arm compared with the HTO arm at 6 weeks and 6 months. Marker or bar heights represent means, error bars indicate SD, and * indicates a statistically significant difference between the arms (P < 0.05).

Figure 5.

(A) WOMAC function ratings decreased in both arms after treatment. Function scores in the ISA arm (green) were significantly lower than those in the HTO arm (blue) at 6 weeks and 3, 6, and 24 months. (B) The percentage of pain improvement at each of the follow-up time points followed a similar trajectory. (C) Responder rates were significantly higher in the ISA arm compared with those in the HTO arm at 6 weeks and 3 months. Marker or bar heights represent means, error bars indicate SD, and * indicates a statistically significant difference between the arms (P < 0.05).

Figure 6.

Percentage improvements relative to baseline in function and pain for the ISA and HTO arms at 3 months (A) and 24 months (B). Boxplot indicates median (notch) and interquartile range (IQR), with whiskers identifying the highest and lowest data point within 1.5x IQR, and * indicates a statistically significant difference between the arms.