Introduction

Osteoarthritis (OA) represents a leading cause of disability and a growing burden on healthcare budgets. OA is particularly vexing for young, active patients who have failed less invasive therapies but are not yet candidates for arthroplasty. Often, patients suffering in this wide therapeutic gap face a debilitating spiral of disease progression, increasing pain, and decreasing activity until they become suitable arthroplasty patients. An implantable load absorber was evaluated for the treatment of medial knee OA in this patient population.

Joint overload has been cited as a contributor to OA onset or progression. In response, the KineSpring® System (Moximed, Inc, USA) has been designed to reduce the load acting on the knee. The absorber is implanted in the subcutaneous tissue without violating the joint capsule, thus preserving the option of future arthroplasty. The implant is particularly useful for young, active patients, given the reversibility of the procedure and the preservation of normal flexibility and range of motion.

Joint load reduction is effective for alleviating OA pain. Treatment options for joint unloading include braces and HTO, both of which may be impractical for patients. The purpose of the present study was to examine the biomechanical rationale of a practical, partial unloading implant (KineSpring® System, Moximed) for knee OA.

Device durability was tested by cyclically loading bone-implant constructs through simulated use for at least 10 million cycles. Joint load reduction with the implant was quantified by measuring changes in medial and lateral knee compartment loads generated by cadaver knees in simulated gait. Safety of the device was tested by 3, 6, and 12 month follow-up of implants in an in vivo ovine model. Surgical technique and device safety and efficacy were assessed in human clinical studies.

The unloader device survived over 15 million cycles of simulated use without failure. In the simulated gait cadaver model, the unloading device significantly reduced medial compartment (29 ± 13 lbs, p<0.05) and overall knee joint loads during the stance phase of gait testing but did not significantly increase lateral compartment loading. Chronic ovine implants demonstrated good tolerance of the implant with normal wound healing and secure device fixation. Clinical experience (n=49) demonstrated uneventful device implantation. Unlike HTO, the implantation technique for the unloader does not alter joint alignment. This surgical technique avoids removal of bone, ligament, and cartilage, thus preserving future primary arthroplasty, if required. Early-term clinical experience also demonstrates good outcomes for patients, the earliest of whom are beyond 2.6 years with the implant.

This unloading device offers a practical and attractive treatment option for patients with medial knee OA: load reduction without load transfer, durability, preservation of downstream treatment options, safety, and early-term efficacy.

Statement of Purpose

The UK and Australian clinical experience of an implantable load absorber was reviewed for knee OA patients who have exhausted conservative care, but are not ideal candidates for HTO or arthroplasty due to age, activity level, obesity, or disinclination.

Joint load correlates with knee OA incidence, symptoms, radiographic, morphologic and biological changes. Available load modifying therapies are clinically effective but have drawbacks. The KineSpringTM (Moximed Inc), an investigational device, is designed to reduce compartment loads while avoiding the limitations of current treatments. We compare load reductions of braces, HTO and KineSpringTM.

Literature review and experimental data provide compartment load changes for clinically effective knee braces and HTO. Simulated gait testing was completed on four cadaver knees with early-stage OA. Gait was simulated using a cadaver-based kinematic test system that applies motion and loading patterns dynamically to cadaver specimens. Medial and lateral compartment femoro-tibial pressures were measured throughout testing using thin film dynamic pressure sensors (Tekscan, Inc.) placed inframeniscally. Three conditions were tested: no treatment, applied valgus moments to simulate a valgus moment brace, and implanted KineSpring.

Sufficient clinical data exists to support the development of new and novel load modifying therapies for knee OA. Joint load reductions provided by HTO and valgus moment braces provide insight into clinically effective load reduction ranges. Opening wedge HTOs of 5° and 10° are reported to reduce average medial compartment load by 55 N (12 lbs) and 286 N (64 lbs), respectively1. Valgus braces were reported to reduce medial compartment loads an average of 97-280 N (22-63 lbs). From this data we propose a clinically effective load reduction range of 55 to 286N is a valid indicator of the likely clinical success for medial knee load reduction treatments.

Gait simulation was successfully completed in all specimens in all test configurations. The valgus moment brace reduced medial compartment load by 58 ±20 N but did not reach statistical significance. The Kinespring reduced medial compartment load by 129±64 N in comparison to the untreated case, a statistically significant reduction. Neither the KineSpring nor the valgus moment brace caused significant changes in the lateral compartment during stance.

All treatments reduced medial compartment loads. KineSpringTM reduces loads in what we determined to be the clinically effective range. Additional studies and clinical investigations are warranted to determine the ultimate effectiveness of this implant system.

Several attempts have been made to treat medial compartment OA of the knee with mobile spacers. All have met with dismal failure.

This presentation explores the history of attempts to treat OA in the younger knee with mobile spacers and explains why they were all doomed to fail.

Sources of information for this presentation include the published peer reviewed literature, publically available documents, and an insiders view of some of the failed attempts to solve the problem of medial compartment OA with mobile spacers.

All attempts to treat medial compartment OA of the knee with mobile spacers have failed. The unispacer has been a failure with a 60% revision rate at three years. The ABS intercushion had a 100% revision rate at one year and in many cases caused permanent damage to the host knees. The Salucartilage spacer was implanted in one patient only and failed within 48 hours.

Mobile spacers do not work, are never likely to work, and are not indicated for the treatment of medial compartment osteoarthritis of the knee.

This prospective study reviews the two-year results in a series of total hip replacements using the Silent Hip ‘neck only’ prosthesis.

The Silent Hip was used in 15 hips in 14 patients in combination with large bore metal heads and ASR metal cups. Results were analysed using pre-operative and two-year Harris hip scores, Oxford hip scores, and UCLA activity ratings. Radiographs were independently analysed for evidence of implant migration, loosening, and bony hypertrophy.

All patients had successful outcomes at two years. The mean Harris hip score improved from 52 to 97.4, with nine patients (10 hips) scoring 100. There were 14 excellent hips and one good hip according to the Harris scoring system. The mean Oxford hip scores improved from 36.8 to 13.8, with nine patients achieving a perfect score of 12. The average UCLA activity rating improved from 5.2 to 8.1.

Independent radiographic analysis demonstrated no cases of implant migration or bony hypertrophy. The only complication in the series was one DVT.

The Silent Hip is a safe and effective implant in patients with adequate femoral neck bone stock.

The advantages of unicompartmental knee arthroplasty (UKA) include its bone preserving nature, lower relative cost and superior functional results. Some temporary pain has been reported clinically following this procedure. Could this be related to bone remodeling? A validated bone remodeling algorithm may have the answers…

A 3D geometry of an intact human cadaveric tibia was generated using CT images. An all poly unicompartmental implant geometry was positioned in an inlay and onlay configuration on the tibia and the post-operative models created. An adaptive bone remodeling algorithm was used with finite element modeling to predict the bone remodeling behavior surrounding the implant in both scenarios. Virtual DEXA images were generated from the model and bone mineral density (BMD) was measured in regions of interest in the AP and ML planes. BMD results were compared to clinical results.

The bone remodelling algorithm predicted BMD growth in the proximal anterior regions of the tibia, with an inward tendency for both inlay and onlay models. Looking in the AP plane, a maximum of up to 7% BMD growth was predicted and in the ML plane this was as high as 16%. Minimal BMD loss was observed, which suggests minimal disturbance to the natural bone growth following UKA.

Positron emission tomography (PET) scans showed active hot spots in the antero- medial regions of the tibia. These results were consistent with the finite element modeling results.

Bone remodeling behavior was found to be sensitive to sizing and positioning of the implant.

The adaptive bone remodeling algorithm predicted minimal BMD loss and some BMD growth in the anterior region of the tibia following UKA. This is consistent with patient complaint and PET scans.

Since 1981, during operations for spinal deformity, we have routinely used electrophysiological monitoring of the spinal cord by the epidural measurement of somatosensory evoked potentials (SEPs) in response to stimulation of the posterior tibial nerve. We present the results in 1168 consecutive cases. Decreases in SEP amplitude of more than 50% occurred in 119 patients, of whom 32 had clinically detectable neurological changes postoperatively. In 35 cases the SEP amplitude was rapidly restored, either spontaneously or by repositioning of the recording electrode; they had no postoperative neurological changes. One patient had delayed onset of postoperative symptoms referrable to nerve root lesions without evidence of spinal cord involvement, but there were no false negative cases of intra-operative spinal cord damage. In 52 patients persistent, significant, SEP changes were noted without clinically detectable neurological sequelae. None of the many cases which showed falls in SEP amplitude of less than 50% experienced neurological problems. Neuromuscular scoliosis, the use of sublaminar wires, the magnitude of SEP decrement, and a limited or absent intra-operative recovery of SEP amplitude were identified as factors which increased the risk of postoperative neurological deficit.