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. Author manuscript; available in PMC: 2022 Sep 1.
Published in final edited form as: Am J Sports Med. 2021 Aug 4;49(11):2933–2941. doi: 10.1177/03635465211028566

Synthetic PVA Osteochondral Implants for the Knee Joint

Mechanical Characteristics During Simulated Gait

Tony Chen †,‡,*, Caroline Brial †, Moira McCarthy §, Russell F Warren ‡,§, Suzanne A Maher †,‡
PMCID: PMC9092221  NIHMSID: NIHMS1795728  PMID: 34347534

Abstract

Background:

Although polyvinyl alcohol (PVA) implants have been developed and used for the treatment of femoral osteochondral defects, their effect on joint contact mechanics during gait has not been assessed.

Purpose/Hypothesis:

The purpose was to quantify the contact mechanics during simulated gait of focal osteochondral femoral defects and synthetic PVA implants (10% and 20% by volume of PVA), with and without porous titanium (pTi) bases. It was hypothesized that PVA implants with a higher polymer content (and thus a higher modulus) combined with a pTi base would significantly improve defect-related knee joint contact mechanics.

Study Design:

Controlled laboratory study.

Methods:

Four cylindrical implants were manufactured: 10% PVA, 20% PVA, and 10% and 20% PVA disks mounted on a pTi base. Devices were implanted into 8 mm–diameter osteochondral defects created on the medial femoral condyles of 7 human cadaveric knees. Knees underwent simulated gait and contact stresses across the tibial plateau were recorded. Contact area, peak contact stress, the sum of stress in 3 regions of interest across the tibial plateau, and the distribution of stresses, as quantified by tracking the weighted center of contact stress throughout gait, were computed for all conditions.

Results:

An osteochondral defect caused a redistribution of contact stress across the plateau during simulated gait. Solid PVA implants did not improve contact mechanics, while the addition of a porous metal base led to significantly improved joint contact mechanics. Implants consisting of a 20% PVA disk mounted on a pTi base significantly improved the majority of contact mechanics parameters relative to the empty defect condition.

Conclusion:

The information obtained using our cadaveric test system demonstrated the mechanical consequences of femoral focal osteochondral defects and provides biomechanical support to further pursue the efficacy of high-polymer-content PVA disks attached to a pTi base to improve contact mechanics.

Clinical Relevance:

As a range of solutions are explored for the treatment of osteochondral defects, our preclinical cadaveric testing model provides unique biomechanical evidence for the continued investigation of novel solutions for osteochondral defects.

Keywords: knee, articular cartilage, biomechanics of cartilage, cartilage repair, hydrogel, implant

Focal articular cartilage defects affect approximately 16% of the general population and 36% of the athlete population22 and pose a serious socioeconomic problem in the form of painful joints that are at high risk for the development of osteoarthritis.9,10 In simplified biomechanical models, focal defects result in an increase in force concentration at the defect rim,21,24 which has been hypothesized to be a key mechanism in the spread of tissue damage. Current clinical treatment methods, such as microfracture, osteochondral autografts and allografts, and autologous cell implantation,14,39 are prone to failure due in part to differences in material properties between the repair and host tissue and difficulties with achieving robust host integration.29 Therefore, there has been recent interest in developing bioreactor grown tissues,2,13,28,40,42 biphasic scaffolds,30,54 and nondegradable implants3,43–45,50 for articular cartilage repair. However, these approaches are prone to failure if they cannot improve the detrimental contact mechanics associated with defects.

Polyvinyl alcohol (PVA) is a widely studied material for chondral and osteochondral defect repair1,3,34,43,51,53 because it is biocompatible,3,45 has controllable mechanical properties,3,26,31 has a low wear rate and coefficient of friction,7,12,36,55 and can be chemically modified to alter its physical characteristics.6,11,27,50,53 However, solid PVA implants have not had success in the repair of osteochondral defects of the knee. In animal models, these have led to fibrous encapsulation and loosening of the implants.33,37 Clinically, use of PVA implants for the repair of osteochondral defects of the knee has led to removal of the implants because of loosening or continued pain.34,47,51 Two approaches have been studied to counteract these problems: (1) use of a porous titanium (pTi) metal base, attached to a disk of PVA, enabling bony integration,45 and (2) use of a solid cylindrical PVA with a high polymer content (ie, stiff), which is held in place through joint compression.58 However, the effect of PVA stiffness or the presence/absence of a porous metal base on joint contact mechanics has not been established. We previously developed a dynamic model capable of applying multidirectional physiological loads mimicking gait across human cadaveric knees.4,23,56 By combining this model with contact stress sensors, we can quantify the effect of focal defects and devices intended to replace those defects on knee joint contact mechanics.

The objective of this study was to quantify the contact mechanics during simulated gait of (1) focal osteochondral defects and (2) PVA cylinders (10% and 20% PVA) and PVA disks mounted on pTi bases. We hypothesized that PVA implants with a higher modulus combined with a pTi base would significantly improve defect-related knee joint contact mechanics.

METHODS

Cadaveric Knee Preparation

Approval for use of cadaveric specimens was obtained from the institutional review board of the Hospital for Special Surgery. Eight cadaveric knees (age, 52.5 ± 14.6 years; 4 female, 4 male) (for individual specimen data, see Appendix Table A1, available in the online version of this article) were selected for experimental testing using the following exclusion criteria: no history of osteoarthritis, diabetes, or traumatic injuries. The knees were prepared and potted into the simulator as previously described by Gilbert et al.23

Simulator Setup

A Stanmore Knee Simulator was modified with custom-designed fixtures to accept cadaveric knees,4,23 as depicted in Figure 1A.

Figure 1.

Figure 1.

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Experimental cadaveric setup. (A) A Stanmore knee simulator was adapted to apply dynamic simulated gait to cadaveric knees. (B) A Tekscan sensor was sutured into place on the tibial plateau of the knees to measure changes in contact stress. (C) Flexion, axial force, anterior-posterior force, and internal-external torque are controlled inputs during the gait cycle, (D) which extends through the stance and swing phase.

Contact Stress Measurement

Contact stress normal to the surface of the tibial plateau was measured using a thin electronic sensor (Tekscan 4011) (Figure 1B) placed on the tibia under the meniscus as previously described by Gilbert et al.23 Each sensor array consists of sensing elements, hereafter called sensels.

Knee Testing

A Stanmore Knee Simulator was used to control the axial force, anterior-posterior force, internal-external torque, and flexion-extension profiles (Figure 1C) to mimic the activity of walking (Figure 1D), as per the guidelines from the International Organization for Standardization (No. 14243–1).18 The knees were first tested intact. Subsequently, the knees were flexed, and a defect was created on the medial femoral condyle using an 8-mm Arthrex low-profile reamer to a depth of 10 mm. The location of the defect was standardized using a series of measurements of the femur and medial condyle (Figure 2A), which was determined in 3 pilot knees. Implants were inserted into the defect in a randomized order, and knees were subjected to simulated gait. The empty defect condition was always the final tested condition to protect the sensor from failing because of high shear stresses caused by the edge of the empty defect.

Figure 2.

Figure 2.

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(A) Standardization of osteochondral defect location. A horizontal line (I) was drawn across the width of the femur tangential to the intercondylar notch. A second line (II), perpendicular to I, was drawn down the medial condyle 0.26 the length of I. The third line (III) was drawn perpendicular to II with a length that was 0.18 the length of I. (B) Schematics of the 4 polyvinyl alcohol (PVA) implants tested. Implants either were composed of PVA only or included a porous titanium (pTi) base at a PVA concentration of 10% or 20%. Photographs of (C) PVA only and (D) PVA + pTi implants.

Implant Manufacture

PVA implants were created using 10% wt/vol or 20% wt/vol PVA to generate constructs with elastic moduli of 40 and 140 kPa, respectively,50 without and with a pTi base (Figure 2). Implants were created by pouring either 10% or 20% wt/vol liquid PVA into 100-cm2 tissue culture dishes. The PVA then underwent 6 freeze/thaw cycles to create constructs with maximum mechanical properties for each concentration.26 After the completion of the freeze/thaw cycles, 8 mm–diameter implants were cored from the polymerized PVA in the dishes. For implants consisting of PVA alone, the implants were shaved to 10.2 mm in height using a sledge microtome. For implants consisting of PVA + pTi, liquid PVA was added to the surface of the pTi and the PVA implant was attached to the pTi. The pTi had a diameter of 8 mm and height of 8 mm with a porosity of 56%, with pore sizes ranging from 80 to 350 μm (median, 150 μm). Young modulus of the pTi was 6 GPa,35 which is within the range of bone.48 The PVA + pTi implant then underwent an additional 3 freeze/thaw cycles, after which the implants were frozen and shaved to a final height of 10.2 mm.

Data Analysis

Medial tibial plateau contact data were collected from 7 of the 8 knees; 1 knee fractured during testing. Because peak axial loads are experienced across the knee joint at 14% and 45% of the gait cycle, we focused much of our analysis on those 2 points and computed contact data after averaging 6 cycles of gait leading to the second to last cycle.5 Regions of interest (ROIs) were manually identified as follows: (1) an ROI within the defect, (2) an ROI under the meniscal footprint (M-C), and (3) an ROI where cartilage-to-cartilage contact occurred (C-C). The defect ROI was manually identified as the area of maximum stress loss on the plateau, regardless of whether that occurred at 14% or 45% of the gait cycle. The C-C ROI was identified at 14% of the gait cycle, while the M-C ROI was inverted to include the remaining sensels on the medial plateau. The ROIs were fixed throughout the gait cycle and between test conditions. Contact area, sum of stress (Σσ), and peak stress acting in each ROI were computed.8,23 The weighted center of contact stress (WCoCS) on the medial plateau was calculated as previously described by Gilbert et al23 for the medial-lateral and anterior-posterior directions for each knee with the defect and with each PVA implant subtracted from the intact WCoCS for each knee. Principal component (PC) analysis was used to reduce the dimensionality of the WCoCS in both the medial-lateral and the anterior-posterior directions.8 Only the first PC (PC1) was used for analysis; in each test, PC1 alone was able to account for >75% of the variation in the data, thus making it a reliable indicator of the differences between each test condition.

Statistical Analysis

The conditions were compared using a paired 2-way analysis of variance (ANOVA) to identify differences in contact area and total contact force between the tested conditions within the defect and at 14% and 45% of gait. Tukey post hoc testing was performed to determine differences between conditions. Paired 2-way ANOVA was also used to identify differences in the PC analysis of the WCoCS, with Tukey post hoc testing performed to determine differences between conditions. The difference in the means was considered significant when P < .05.

RESULTS

Of the 7 knees analyzed, 5 had defects with a mechanical effect in the C-C region and 2 had defects with a mechanical effect in the meniscal footprint region (Figure 3). The defects were apparent at 14% and 45% of the gait cycle in 6 knees, while in 1 knee the defect could be identified only at 45% of gait (Figure 3, knee 1).

Figure 3.

Figure 3.

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Images of all specimens at (A) 14% and (B) 45% of the gait cycle, with white circles illustrating the region opposite the defect as identified by the sensor. Knee 2 fractured during testing and was not included in the analysis.

Defect ROI

The defect resulted in a significant decrease in contact area, peak stress, and sum of stress relative to intact condition (Figure 4A). All implants, except for the 10% PVA, resulted in a significant increase in contact area relative to defect condition, and there was no significant difference between the contact area of all implants versus intact. All implants had a significantly lower peak stress compared with the intact condition. All implants resulted in a significantly lower sum of stress versus intact, except for the 20% PVA + pTi, which was not significantly different from intact.

Figure 4.

Figure 4.

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Measurements of contact area, peak stress, and sum of stress in the (A) defect, (B) cartilage-to-cartilage, and (C) under the meniscal footprint regions of interest (ROIs). Defect ROIs were computed at either 14% or 45% of the gait cycle, depending on when in the simulated gait cycle the defect was mechanically active. The mean and SE are shown. *Significant difference between conditions (P < .05). The Tekscan images are from knee 4 at 14% of gait, with the red outline representing the drawn ROI and the red highlighted areas representing the sensels identified within the ROI. pTi, porous titanium; PVA, polyvinyl alcohol.

Cartilage-Cartilage ROIs

The contact area in the C-C zone at 14% and 45% of the gait cycle was not significantly affected by any condition (Figure 4B). No differences were seen in peak stress between conditions. The sum of stress was significantly lower than the intact condition for 20% PVA at 14% of the gait cycle. At 45% of the gait cycle, the sum of stress was significantly lower than the intact condition for the defect and all PVA devices. The 20% PVA + pTi led to a significant increase in sum of stress versus the defect condition.

Meniscal-Footprint ROI

At 14% of the gait cycle, contact area in the M-C zone was not affected by any condition (Figure 4C). At 45% of the gait cycle, the contact area in the M-C zone was significantly higher for 10% and 20% PVA than the intact condition; no other differences occurred. No differences were noted in peak stress in the M-C zone. At 14% of the gait cycle, the sums of stress in the M-C zone were significantly higher for the defect condition versus intact. Implantation of the 20% PVA + pTi led to a significant decrease in the M-C sum of stress versus the defect condition at 14% of the gait cycle; regardless of implant type, PVA implantation had no effect on the sum of stress at 45% of the gait cycle.

Weighted Center of Contact Stress

No differences were found when comparing PC1 for the medial-lateral WCoCS (P > .97) (Figure 5A). In the anterior-posterior direction, significant differences were found between the PC1 for the defect (P = .02), 10% PVA (P = .02), 20% PVA (P = .009), and 10% PVA + pTi (P = .01) conditions compared with intact (Figure 5B). No difference was noted between the intact condition and 20% PVA + pTi (P = .08).

Figure 5.

Figure 5.

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Weighted center of contact stress (WCoCS) first principal component (PC1) plots. The PC1 values for the (A) medial-lateral WCoCS and (B) anterior-posterior WCoCS are shown for each condition. Each point represents the PC values for each specimen tested, with the 95% CIs represented by the error bars. In all plots, the dotted line represents the intact condition. *Significant differences from intact (P < .05). N.S., significant difference from intact; pTi, porous titanium; PVA, polyvinyl alcohol.

DISCUSSION

By way of a dynamic, multidirectional simulator, we quantified the contact mechanics during simulated gait of an 8-mm femoral focal osteochondral defect and several differently designed PVA implants. By quantifying the contact area, peak stress, and sum of stress changes in well-defined ROIs across the tibial plateau at 14% and 45% of the gait cycle and the WCoCS tracked throughout the gait cycle, we accepted the hypothesis that PVA implants with higher modulus (20% PVA) and augmented with a porous metal base restored contact mechanics more closely to that of the intact knee compared with other designs. The addition of a pTi base not only provides improved contact mechanics but also may prevent material mismatch between the implant and the bone, which can cause fibrous encapsulation37 and loosening of the implants.34,51 Further, the previously reported osteoinductive nature of pTi49 will likely provide long-term stability to the implants.

Repair of osteochondral defects is crucial for the continued health of the cartilage, as it has been posited that high rim stresses lead to further degradation of the cartilage surrounding the defect, eventually progressing to osteoarthritis. While tissue engineering2,13,28,40,42 and scaffold30,54 approaches have created tissues with properties that are similar to cartilage upon implantation, in vivo outcomes have been poor because of the catabolic environment of the injured joint that can cause failure of the repair.46

Synthetic implants, such as those made of PVA, have the theoretical advantage of being able to prescribe implant mechanical properties, to control force distribution on the cartilage surface, without succumbing to changes due to the environment. However, there have been complications related to the clinical outcome of PVA implants. Sciarretta51 used PVA implants in 18 patients, with 3 patients requiring revision (2 to total knee and 1 to osteochondral autografts). Lange et al34 implanted PVA hydrogels in 18 patients with stage 4 chondral lesions. These patients had decreases in patient-reported outcomes after 12 months, with failures attributed to poor fixation of the implants within the defects. More recently, de Queiroz et al17 implanted 38 patients with either PVA implants or osteochondral autografts with follow-ups at 6, 12, and 24 months. Two patients in each group had prolonged pain that decreased at 1 year. While this study showed promise in the use of PVA implants for the treatment of chondral and osteochondral defects, a longer follow-up needs to be performed to understand their long-term performance. Furthermore, while not significantly different, the pain scores in the PVA group were higher, which could be attributed to the inability of the implants to fully restore mechanics, along with mismatch in mechanics between the PVA and bone.

The ability of implants to mechanically function within the knee joint should ideally form part of the preclinical evaluation process,38 but because of the complexity of creating cadaveric models that can mimic physiological loads, such evaluations are rare. Sismondo et al53 used a cadaveric model that applied one-third of physiological forces to quantify the ability of a PVA hydrogel implant to restore the stress distribution with a focal articular cartilage defect. Other studies have included an assessment of scaffold fixation during simulated continuous passive motion20 and analyses of the effect of osteochondral graft height on contact mechanics.32 The analysis of the contact forces during simulated gait with the full magnitude of physiological applied loads, the tracking of the WCoCS, and the analysis of the regional effect of the defect and various implant designs are novel.

While understanding changes within the defect provides insight into the load-bearing ability of the implant, analysis of the C-C and M-C regions elucidates the restoration of overall joint function. Our data on the effect of osteochondral defects on contact mechanics align with data reported by others from static cadaveric models and computational finite element models.9,24,47 Guettler et al24 observed a size threshold above which stress concentrates at the rim of the defect; this stress localization was observed for defect sizes 10 mm in diameter and larger, while Brown et al9 observed the highest force concentrations in 2 mm–diameter defects in a canine model and a decrease in stress concentration with increasing defect size. This defect diameter studied by Brown et al corresponds to a relative size of 25% of the condylar width, which is similar in relative size to the defects used in the current study (20%). By testing the effects of osteochondral defects in a dynamic multidirectional simulator system, we could quantify the changes that occur over the gait cycle across a range of flexion angles. While we did not note any increase in peak stress at the edge of the defects, we observed that the presence of a defect in areas of C-C contact caused redistribution of contact force across the entire plateau, including across the meniscal footprint. This result is similar to a finite element model of focal articular cartilage defects,47 where defects >1 cm2 had a large effect on rim forces, and meniscal removal caused an increase in rim forces regardless of the size of the defect. This finding suggests an unexplored relationship between the health and condition of the meniscus and mechanical consequences of osteochondral defects, which can affect the mechanics of the entire joint. This relationship is particularly important as osteochondral defects are common comorbidities to meniscal injury.15,57

Our purpose in creating 4 different implants was to emulate clinically used PVA implants that are single material implants. These implants do not allow for bone ingrowth or ongrowth,33 whereas hydrogels attached to porous metal bases allow for bone ingrowth.31,44,45 We intentionally manufactured the PVA to span 2 different polymer content values, within the range of that used clinically, leading to compressive moduli of 0.07 ± 0.01 MPa for the 10% PVA and 0.33 ± 0.02 MPa for 20% PVA (as quantified during unconfined compression testing). While these values are low compared with native cartilage (6.3 ± 3.59 MPa), when confined in a defect, the instantaneous modulus of 20% PVA hydrogels is similar to the instantaneous modulus of native cartilage.50 Finally, all implants were placed 0.2 mm proud, which improves the load-bearing capabilities of the implants19,25 and avoids potential damage to opposing tissue in versions that have been kept as much as 1 mm proud.53

Of all implants tested, 20% PVA + pTi resulted in no significant difference in the sum of forces in the defect ROI when compared with the intact condition; a significant decrease in the sum of forces in the M-C ROI versus defect at 14% of the gait cycle; and a significant increase in the sum of forces in the C-C ROI versus defect at 45% of the gait cycle. In addition, the WCoCS in the anterior-posterior direction was not significantly different from the intact condition for the 20% PVA + pTi device. The WCoCS is an indicator of the center of loading, and alterations in the location of the WCoCS suggest that there is a shift in the distribution of the stress. These results indicate that across all metrics, the 20% PVA + pTi has advantageous load-bearing properties. The mechanism driving the behavior of the 20% PVA + pTi implant in the joint is likely multifactorial: a rigid base in combination with a higher modulus polymer may more closely mimic the native tissue. However, further studies are required to quantify the relevance of differences in contact mechanics and their effect on the biological response of the articular cartilage.

We must take into account several limitations in the interpretation of these results. First, the simulator system does not control loading in the medial-lateral or varus-valgus directions. It would be important to test the effects of varus and valgus joint loading on the contact mechanics in the presence of a defect and with the different repairs; in normal gait kinematics, there is a bias toward varus joint load-ing.41 Second, the electronic sensor can only measure contact stresses perpendicular to the articular surface. While the compressive load is an important predictor of future cartilage damage,16 another important factor in predicting early-onset osteoarthritis is the changes in shear stress between the 2 opposing contact surfaces,52 which cannot currently be measured in this system. Additionally, we did not quantify the shear strength of the interface between the PVA and pTi. For such dissimilar materials, quantification of shear properties will be critical before clinical use. Finally, while the location of the defect was standardized, the defect diameter was held constant irrespective of the specimen condylar width. While this changes the relative size of the defects, 8-mm (<0.5-cm2) defects are a common size observed in patients.57

Despite limitations, the information obtained using our cadaveric test system provides biomechanical support to further pursue the efficacy of high-polymer-content PVA disks attached to a pTi base for the treatment of osteochondral defects.

Supplementary Material

Supplementary files

Footnotes

One or more of the authors has declared the following potential conflict of interest or source of funding: Research reported in this publication was supported by the National Institute of Arthritis and Musculoskeletal and Skin Diseases, part of the National Institutes of Health (NIH), under award No. R01 AR057343. Funding was also received from KL2RR000458 of the NIH-funded Clinical and Translational Science Center at Weill Cornell Medical College. T.C. is an employee and has ownership interest in AGelity Biomechanics. M.M. has received education support from Gotham Surgical Solutions. R.F.W. has received royalties from Arthrex and Zimmer Biomet and has ownership interest in AGelity Biomechanics, CyMedica Orthopedics, OrthoSensor, Stryker, Trice Medical, and Zipline Medical. S.A.M. has ownership interest in AGelity Biomechanics. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. AOSSM checks author disclosures against the Open Payments Database (OPD). AOSSM has not conducted an independent investigation on the OPD and disclaims any liability or responsibility relating thereto.

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