The key goal for the initial resection is to obtain a flat surface to mount the sensor. A resection of 6mm can be competed using the Cutting Guide if the patella is thin. Estimated bone loss can be accommodated for in this initial resection. A guide to the bone loss can sometimes be the medial unworn side of the patella. A significantly reduced resection may require a freehand cut or other instrumentation.

To reduce patellofemoral joint forces, alternative options include:

Using a smaller patella implant, to reduce the construction thickness without having to additionally resect the patella below a thickness of 12mm. A surgeon could check if a smaller patella implant would reduce overload by trialling thinner shims on the sensor.

Repositioning the implant medially or laterally, to prevent overloading in specific areas.

Bone removal at the patella margin, can be used to make small changes to the thickness of the patella at certain points, without increasing the risk of patella fracture by resecting across an entire plane of the patella.

The “natural” reading reflects the preoperative force in the patellofemoral joint before femoral and tibial resurfacing. The device acts as a comparator and informs you of the change in patellofemoral joint force that positioning the femoral and tibial components has inflicted.

In addition, many patients come into surgery with medial or lateral knee pain rather than anterior knee pain. Postoperatively, anterior knee pain may occur due to disruptions in the quadriceps mechanism during the procedure. Therefore, the initial reading informs you of the force through the patellofemoral joint when the patient does not have anterior knee pain.

The forces through the patellofemoral joint are different across all patients, and the impact of 1mm thickness will be different on a patient-by-patient basis. In the literature, a reduction of 2mm in patellar thickness has been shown to decrease loads intraoperatively[See reference]. With Quadsense, the effect of changing the thickness by 1mm can be tested using shims, which come in 1mm increments and with different angles (0° and 2.5°) as part of the system.

[Reference: Tanikawa, H., Tada, M., Ogawa, R., Harato, K., Niki, Y., Kobayashi, S., & Nagura, T. (2021). Influence of Patella thickness on Patellofemoral pressure in total knee Arthroplasty. BMC Musculoskeletal Disorders, 22(1). doi:10.1186/s12891-021-04175-y]

While different incision types can impact the load, if the incision type and length does not change between the sensor readings then the device will perform its function as a comparator and inform the user of any changes in force due to implant positioning. Therefore, it is critical to not alter the incision length during the Quadsense procedure and introduce another variable. The changes in load from the incision are generally less significant than the load changes resulting from femoral and tibial resurfacing. In flexion, stability of the patella is provided by the mechanical articulation between the patella and femoral condyles rather than solely by the retinaculum and skin.

The variable that Quadsense is measuring, the force through the patellofemoral joint, is independent of the articulation of the shim. Quadsense does not provide information on the forces that the implant would experience, and therefore does not aim to replicate its shape or exact position. The Quadsense and shim collectively act as a “digital spacer block”. This design allows the Quadsense device to isolate changes in load due to femoral and tibial resurfacing.

Better informed resections of the patella can help reduce patellofemoral loads and alleviate some symptoms of overstuffing in most cases, but there may be instances where improved patellar resection alone isn’t enough. Using Quadsense provides the opportunity to adjust the patella to alleviate symptoms of overstuffing in most cases.

The data presented at the Knee Society meeting demonstrated that increased patellofemoral pressure, hence overstuffing, occurs in 34%[See reference] of all total knee arthroplasty (TKA) patients, with a significant minority experiencing more severe overstuffing. This overstuffing typically results from the resurfacing of both the femur and tibia, which disrupts the natural relationship between the patella and femur, as the focus is often on balancing the flexion-extension gaps.

However, further resection of the patella has been shown to reduce patellofemoral loads. Additionally, using a smaller patella button can help manage these loads by providing a thinner implant profile. Bone removal from the margin of the patella further contributes to reducing patellofemoral pressures, improving implant fit and optimising load distribution across the joint.

[Reference: Li, C. Y., Ng Cheong Chung, K. J., Ali, O. M. E., Chung, N. D. H., & Li, C. H. (2020). Literature review of the causes of pain following total knee replacement surgery: prosthesis, inflammation and arthrofibrosis. EFORT Open Reviews, 5(9), 534–543. doi:10.1302/2058-5241.5.200031]

Not yet. Currently our data, presented at both European and American knee society conferences, shows significant changes in loads following femoral and tibial resurfacing. Approximately 34% of patients are overstuffed[See reference 1], with 20% being significantly overstuffed[See reference 2], resulting in up to a 200% increase in loads in some cases[See reference 3].

We believe there are many advantages to the Quadsense system, Including improving the state of the art in Patella resurfacing from a technical perspective, showing over stuffing intraoperatively and allowing the surgeon to manage it, and ultimately improving the outcomes in TKR by helping to reduce the incidence of anterior knee pain postoperatively.

[References:

1. Li, C. Y., Ng Cheong Chung, K. J., Ali, O. M. E., Chung, N. D. H., & Li, C. H. (2020). Literature review of the causes of pain following total knee replacement surgery: prosthesis, inflammation and arthrofibrosis. EFORT Open Reviews, 5(9), 534–543. doi:10.1302/2058-5241.5.200031
2. Kahlenberg CA, Nwachukwu BU, McLawhorn AS, Cross MB, Cornell CN, Padgett DE. Patient Satisfaction After Total Knee Replacement: A Systematic Review. HSS J. 2018;14(2):192-201. doi:10.1007/s11420-018-9614-8
3. Patellofemoral Syndrome Clinical Presentation, Buday CM, Stephenson RO, Physical Medicine and Rehabilitation, Jun 17; 2024, https://emedicine.medscape.com/article/308471-clinical#showall, Accessed 19 Nov 2024]

The additional time includes the time it takes to make the two readings (two readings of 12 seconds each), the additional patella resection required to be conducive to a balanced force through the patellofemoral joint.

In cases where there is a significant load change there can be additional added time to take sensor readings with different shim thickness, angulation or position in order to balance the patellofemoral joint force which will inform anu further patella resection or implant positioning. This process will only add a couple minutes onto the procedure and will not significantly increased surgery duration. When there is minimal change in load, the Quadsense system adds negligible time to the procedure.

Placement of the sensor in an apex-dominant position is recommended and must only be used if the sensor is in an identical position for the two comparative readings. This is why the surgical technique recommends marking the position of the sensor for the ‘native’ reading to ensure identical placement for the ‘TKA’ reading. The sensor handle

should be placed perpendicular to the femur/tibia, to ensure that the four sensors (L, M, S, I) are in the correct place to report accurate readings.

For a thin patella, where additional resection is not an option, medialising the sensor position can be a way of reducing high loads and can precede medialised implant placement.

With a Sub-vastus approach, resecting the patella and leaving it in-situ could risk damage to the cancellous bone of the patella. For example, when levering against it with a metal surgical instrument. Therefore, it is recommended to protect the cancellous bone with Shims, or patella protectors common in Sub-vastus procedures.

Based on experience we expect that there would be a difference, but we do not have enough clinical data to make a robust judgement. Quadsense can be used by any surgeon with any alignment philosophy.

The 2.5 degree angle for the angular shim originates about the centre, resulting in the centre of an angled shim and equivalent non-angled shim having the same thickness. The resulting thickest and thinnest outer edge therefore increase and decrease in thickness by the same amount respectively.

The Quadriceps mechanism is used as a spring to measure the changes in geometry because of resurfacing the femur and tibia, so there is no difference if it is passive. The Quadsense device doesn’t provide information on what the active joint reaction force is, it simply acts a comparator and informs the user of what has changed in the joint.

This is consistent with the current way of assessing flexion-extension space.

The cutting guide slot can vary between 1.27mm to 1.37mm.

An additional reading after the final resection is made will not be comparable to readings made before a resection. This is because the position of the sensor pads relative to the patella construct before and after resection is different. If a final resection is made, and the sensor head is installed with a shim which constructively resembles

the original conditions, the reading is still unable to be used as a valid comparative reading.

The range of Shims is provided to allow a surgeon to trial if additional resection would be successful in reducing the PFJ forces according to the Quadsense sensor readings before the additional resection is made.

This can be approached on a patient-by-patient basis. If osteophytes hinder the cutting guide or sensor from being used correctly or proper flexion of the knee, they can be removed before. However, if they are judged as non-intrusive by the surgeon, they can be removed following a decision on the final size of the implant and additional patella resection after Quadsense has been used.

The current literature concludes that instances of anterior knee pain are the same regardless of whether the patella has been resurfaced[4], therefore it is not the resurfacing or lack of resurfacing that is producing instances of pain. Our data suggests that it is changes in load caused by femoral and tibial resurfacing that contribute significantly to anterior knee pain. As the changes in load are unpredictable, by resurfacing you are provided with the opportunity to adjust the changes in load.

[Reference: https://pubmed.ncbi.nlm.nih.gov/36479594/, Tang X, He Y, Pu S, et al. Patellar Resurfacing in Primary Total Knee Arthroplasty: A Meta-analysis and Trial Sequential Analysis of 50 Randomized Controlled Trials. Orthop Surg. 2023;15(2):379-399. doi:10.1111/os.13392]

The shims can be attached to the sensor according to the four cardinal positions with the thinner side of the shim located medial, lateral, inferior or superior.

Early evidence suggests that it does make a difference, as you would expect. The PS knee drives the kinematics, and the CR knee allows the patient’s soft tissues to move freely. The Quadsense device can be used on both cases, regardless.

Each sensor is individually calibrated in production, and this is essential for device performance. The standard reusable sterilisation process would negate the calibration, therefore the device is single use. We are exploring opportunities to reduce the environmental impact and reduce the use of single-use plastics.

If correct cementing technique is adhered to, the implants are designed to accommodate cement by the inclusion of a cement pocket on the implant’s reverse side.

Each patient’s load is unique to them, and unpredictable. We do not yet have data on what a perfect load would be for a specific patient phenotype. The device provides a baseline measurement which, if the patient does not have anterior knee pain prior to surgery, is a good target to aim for post resurfacing of the femur and tibia.

If a tourniquet is used during the procedure and for the initial sensor reading, all subsequent sensor readings for that patient should take place with the tourniquet up. Changing this in between readings is a variable that could affect the force measured. The device can also be used without a tourniquet but requires the user to proceed without a tourniquet for all sensor readings.

The device has been and can be used with multiple robotic platforms and various assistive technologies successfully.

The device is most useful when used during flexion, at which point the patella is inherently stable as it is fully engaged within the trochlea. This is in line with the current precedent set, that stability during flexion and extension is assessed without closing the medial retinaculum. You can close the retinaculum to take a reading, but this must be done in the same manner for all readings to allow the device to perform as a comparator which allows the Quadsense device to isolate changes in load due to femoral and tibial resurfacing.

Quadsense is a capacitive force sensor. Each sensor is individually calibrated to within a 10% variation.