1 Introduction

There is no doubt that total knee replacement has become a successful, reliable and life changing intervention for many thousands of patients, with established survival figures above 95 % over fifteen years.¹ More recently however, attention has turned to the quality or functionality patients experience after total knee replacement. It appears that up to 20 % of patients experience significant discomfort or pain around their knee that precludes their normal activities of daily living.² Activities such as stair climbing or descent, kneeling or twisting appear to give soft tissue pain, principally around the anterior portion of the knee.^3,4 The concentration of pain and discomfort around the anterior structures of the knee has led researchers and surgeons to focus primarily on these tissues that make up the extensor mechanism of the joint and the surrounding stabilising structures, collectively called the “third space of the knee”.

2 The third space

The name third space derives from the historic focus of knee surgeons to the first and second spaces of the joint, namely the flexion and extension gaps of the tibiofemoral joint. Balancing these two spaces is key to achieving stability and function in a total knee replacement and is accepted as part of the surgical process for total knee implantation. The third space is now becoming established as an important part of knee surgery particularly in its role in determining the functionality and satisfaction of the patient post-surgery.

Anatomically, the third space consists of the extensor hood, the patellofemoral joint, the medial and lateral retinacula fibres, the surrounding soft tissues and the quadriceps muscle. These tissues and the synovium within the capsule of the joint are highly innervated and provide the patient with significant sensory feedback both for pain and function during the activities of daily life,^5,6 and abnormal tissue tension in the third space following knee replacement has been implicated in the poor functional outcome in some patients.⁷ It is established that surgeons will routinely balance the first and second spaces (flexion and extension gaps), more recently it is hoped also they will balance the third space in order to address the significant proportion of patients who have compromised functional results after knee replacement.⁸

3 Patello-femoral mechanics

Patello-femoral mechanics are complex and variable dependent on the activity undertaken and are a result of a combination of anatomical bony structures and the surrounding soft tissue and muscle structures that encircle the mobile Patella.^9,10 As patello -femoral function is a dynamic 3-dimensional moving concern,¹¹ it has only been possible recently to characterise the loading and motion of the third space with instrumented jigs and dynamic computational finite element analysis (FEA).¹² The patello-femoral joint appears to have varying constraint and loading, dependent on the degree of flexion of the joint. At 0–30 degrees of flexion, the patella is lightly constrained by the relatively flat trochlea and is guided by the medial and lateral retinacular ligaments to the deepening trochlear groove, delivering the patella to the top of the intercondylar notch of the knee at about 30°.¹³ At this point progressively onwards, the patella becomes more heavily constrained by anatomical bony features of the intercondylar notch and at the same time becomes more heavily loaded as the knee increases in flexion. The medial and lateral retinacular fibres become relatively loose after 30°. The path or track of the patella in early flexion to 30° is often variable, dependent on factors such as the rotation of the foot at heel strike and the force within the quads muscle. It appears the function of the retinacular ligaments¹⁴ is to guide the patella from its variable starting point in extension to the apex of the intercondylar notch where it will become more heavily constrained and heavily loaded. The variable position the patella must adopt in extension secondary to different quads forces and changing rotation of the leg, is facilitated by the relatively flat low constraint of the trochlea at this point.^15,16(seeFig. 1)

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Fig. 1 Three different possible paths for Patella femoral movement dependent on the starting rotation of the foot. The guiding role of the retinacular ligaments is clear.

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From 30° onwards as the patella becomes progressively more loaded, it has an increasing requirement for higher constraint, and this is provided by the deepening intercondylar groove and the patella track becomes more regularised and predictable.

Understanding these features and the requirement for low constraint in early flexion has led to a redesign of several total knee replacements with a lower, shallower trochlea and with less prominent trochlear shoulders to avoid impinging on the sensitive extensor hood tissues.¹⁷ This understanding has led a move away from the concept of “capturing the patella” that led to designing deeper, unphysiological trochlear grooves for knee replacements in the past.^18,19(see Fig. 2)

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Fig. 2 Redesign of trochlea to reduce soft tissue impingement (yellow highlights older design).

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4 Balancing the third space

Just as surgeons have realised the importance of balancing the 1st and 2nd (flexion and extension) spaces to achieve stability and appropriate soft tissue tension, it is increasingly clear that in order to maintain the function of the extensor apparatus and avoid unphysiological soft tissue tension, the third space should also be balanced in total knee replacement. It is important to maintain the anterior offset which allows the extensor mechanism to generate appropriate extensor power.²⁰(see Fig. 3)

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Fig. 3 Anterior offset allows effective function of the extensor hood.

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Surgeons do not routinely measure the thickness of bone they remove in the anterior cut or trochlea resection during the preparation for knee replacement. However, this is vital in balancing the third space and reproducing the function of the anterior offset without over tightening the sensitive structures around the third space to result in anterior knee pain. The average thickness of the trochlear in a knee replacement is about 6 mm. If surgeons remove 10 mm thickness of host bone from the trochlea and replace it with 6 mm thickness of implanted trochlea, understuffing of the third space has occurred by 4 mm with reduction of the anterior offset. As a result, the quadriceps mechanism will have to work 2.1 times harder to generate similar force. It is thought these patients report fatigue and lack of power when undertaking functional tasks as the quadriceps mechanism has to work harder to generate similar movement.¹⁰ If surgeons remove 2 mm of bone and replace it with 6 mm of metal trochlea, they have overstuffed the third space by 4 mm. This results in 1.7 times more soft tissue tension.²¹

It is believed these patients present with tightness and discomfort on functional activities.

Conversely it has also been shown how the femoral component size and position might alter the contact pressure of both patella-femoral and tibio-femoral joints as well as quadriceps forces which could lead to poor clinical and functional outcomes.²²(see Fig. 4 and 5)

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Fig. 4 Accurate femoral sizing with appropriate trochlea resection in green. Understuffing, or over resection of the trochlea and failure to balance the third space in red.

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Fig. 5 Accurate femoral sizing with appropriate trochlea resection in green. Overstuffing, or under resection of the trochlea and failure to balance the third space in blue.

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5 Operative technique for balancing the third space

In order to avoid over or under stuffing the third space, surgeons must clearly aim to resect the same amount of native host bone as they intend to replace with the thickness of the implant trochlea. The amount of bone to be removed can be estimated prior to resection by using the tibial stylus in the anterior slot of the cutting guide.(see Fig. 6)

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Fig. 6 Using the tibial stylus to measure projected anterior resection.

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The thickness of the trochlea resection is determined by the sizing and positioning of the femoral implant. However, moving the femoral implant anteriorly in order to reduce trochlea resection, will also have an undesirable effect by increasing the flexion space. Equally, moving the implant posteriorly in order to increase trochlea resection will decrease and tighten the flexion space. Surgeons may avoid this unwanted complication by using a “posterior up” method of positioning the femoral cutting block.

Fig. 7 by using the “posterior up” method of pinning the femoral cutting block the posterior resection and flexion space remains constant, whilst the trochlear resection can be determined by changing the femoral size. When the femoral size is decreased to increase trochlea resection there are no issues, however if the femoral size is increased to reduce trochlea resection there is a technical risk of medial to lateral overhang. Therefore, the narrow version of the increased femoral size should be selected (see Fig. 8).

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Fig. 7 Use of posterior pins (purple) to position the femoral cutting guide. Using this method, size changes (ie size 5 to 6) directly affect the trochlea resection, without compromising the flexion space.

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Fig. 8 Patella sensor device for Four quadrant monitoring (Quadsense).

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By this method, surgeons may achieve a balance of the third space without compromising the balance of the first and second spaces.

This technique however, is a compromise based on the limitations of current orthopaedic knee instrumentation primarily designed to cut and balance the 1st and 2nd spaces. The advent of robotic surgery or robotic assisted surgery allows a much higher degree of planning in the pre operative phase and the opportunity to orient the femoral component in three dimensions.²³ Many robotic assisted systems (RAS) allow pre planning to anticipate the effects of different levels of bone resection and femoral implant positioning. Currently, existing models will predict tibio femoral kinematics resulting from femoral prosthetic positioning but are unable to show the resultant patello femoral kinematics.²⁴ Further developments are expected in this sphere to allow a better prediction of the Patella track related to the tibio femoral joint and the effect of femoral prosthetic rotation on patello femoral performance.^25,26

For surgeons who do not resurface the patella, accurate restoration of the trochlear thickness or balancing the third space is achieved with the trochlear resection alone. It appears that within ±2 mm of the native trochlear thickness there is not a discernible effect on the extensor apparatus. However, when the inaccuracy of the resection or failure to balance the third space reaches ±4 mm there is a significant effect on the function and on the soft tissue tension in the third space^21,22,27. Therefore, rather as balancing the first and second spaces, within ±2 mm appears to be acceptable for balancing the third space.

Surgeons who resurface the patella have a second factor to consider in balancing the third space. Most will perform a measured resection of the patella, removing as much patella bone as they intend to replace with the patella prosthesis, i.e., 9-mm resection of the native patella is required for a 9-mm polyethylene button. It is also possible to consider fine tuning or more accurately balancing the third space by changing the depth of resection of the Patella. For instance, if in choosing a femoral size the surgeon realises the third space is 2 mm understuffed they would have the option of under resecting the Patella cut by 2 mm, removing 7 mm rather than 9 mm therefore restoring the balance of the third space perfectly. The option of using the patella resection to balance the third space more accurately has led to a developing interest in trying to record the dynamic forces that occur over the implanted trochlea and to modify the patella resection both in depth and angle to try and reproduce the native pressures in the patello femoral joint. By reproducing the native patella-femoral pressures that existed before knee replacement, it follows that the tension in the surrounding tissues of the third space would also be similar to that before surgery. In this way it is hoped that the anterior knee pain and the loss of function that troubles less than 20 % of knee arthroplasty patients might be addressed.²⁸

The employment of robotics and artificial intelligence algorithms to this fairly complex calculation not only in depth of resection but also in the path of the Patella and the subsequent role of the femoral prosthetic position would be important in adding an element of sophistication and predictability in these otherwise quite simple assumptions.²⁹ Currently with standard technology there is no mechanism of performing some of these predictive calculations and this remains a very exciting sphere of development for robotics.³⁰

6 Dynamic measurement of the intraoperative patello-femoral contact forces

It is clear from our recent understanding of the dynamic and variable nature of the patello-femoral mechanics (described above) that the loading of the patellofemoral joint is extremely changeable and highly dependent on the flexion angle and the loading of the knee. Contact forces will also vary in the different quadrants of the patella articulation.⁹ Unlike other aspects of knee surgery there is not one single metric that accurately describes patellofemoral function. In order to accurately reproduce and balance the third space, detailed information is required regarding the patella femoral loading and the pattern of that loading as the knee flexes. The third space may be balanced in a similar way to the first and second spaces by ensuring resected bone is accurately replaced by prosthetic material.²⁵ However, the dynamic and changing nature of the patella-femoral joint means information regarding the loading of the third space between 0 and 140° would be of benefit if the soft tissue tensions around the third space are to be accurately reproduced throughout the whole range of movements of the knee.

There is not a traditional method to measure this dynamically variable value, digital technology linked with sensor development gives insight to this complex issue.

A four quadrant pressure sensor device for the patella has been developed, which allows accurate recording of patellofemoral pressure throughout the range of movement in the knee (Quadsense, Eventum Orthopaedics Ltd, Ilkley, UK.). The sensor is shaped as the Patella button and contains four pressure sensors in the superior, inferior, medial and lateral sections. The sensor requires a 6-mm resection of Patella bone and is connected to a device outside the operative field to allow the surgeon to study the resultant pressures in each to the quadrants throughout a range of flexion in the knee.

At the commencement of the procedure, the surgeon first makes a 6-mm resection of the Patella and records the pattern, distribution and pressure of patellofemoral forces in the native knee through flexion and extension prior to operative surgery. Laboratory studies and tests in vivo reveal that the patellofemoral forces are very individual and vary between patients but are consistent and reproducible in pattern and amplitude when repeated studies are performed on the same patients.³¹Fig. 9(a).

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Fig. 9a Repeatable pattern of patella-femoral forces through 5 flexion and extension cycles. Quadrants: yellow-inferior, green-medial, red-lateral, blue-superior.

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Knee replacement is then undertaken, and the tibia and femur are implanted with the appropriate tibial polyethylene insert. All efforts are made to complete an accurate trochlea resection. The sensor device is then repositioned on the cut surface of the patella and the knee is flexed and extended again to repeat the measurements now the knee has been replaced. Characteristically the pattern and the amplitude of the pressure measurements are commonly very different following knee replacement (Fig (9b)). The surgeon now has an opportunity to alter the patella resection in both depth and angle in order to reproduce the pattern and amplitude of the native knee recording taken prior to implantation. The aim therefore is to reproduce what existed before in terms of pressure patterns and pressure distribution in order to replicate as accurately as possible the patients physiological third space pressures and movements. In this way it is hoped that the third space function will be maximised, avoiding over tensioning that is caused by over stuffing or loss of function that follows under stuffing.

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Fig. 9b Patello-femoral pressures measured following TKA through 5 cycles of flexion and extension. Quadrants: yellow-inferior, green-medial, red-lateral, blue-superior.

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Fig. 9c Patello-femoral pressures measured after corrective resection of patella to normalise third space pressure. Quadrants: yellow-inferior, green-medial, red-lateral, blue-superior.

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In the patient summarised in Fig. 9a and b and c, the initial assessment of the native state shows mostly patella femoral pressure on the lateral facet in red with a much lesser component in the superior section in blue. The graph records several cycles of flexion and extension, showing that although the patterns are individual, they remain consistent throughout the assessment. Fig. 9b shows the effect of knee replacement with the Patella sensor recording similar lateral pressures in red but now an overpressure of the superior facet (blue) which has risen to equal the pressure in the lateral facet. The surgeon has an opportunity now to address this change and reproduce the patellofemoral pressures that the patient was accustomed to as normal before knee replacement. By taking an extra 2 mm superior angled cut with a special patella cutting guide, the surgeon can address the overpressure on the superior facet (blue) to reduce it to the level which existed in the pre operative state(Fig. 9c). As such, it is hoped that the pressures and tensions of the soft tissues around the third space can be accurately controlled and returned to the preoperative levels perhaps addressing the ongoing feature of anterior knee pain in 20 % of knee replacement individuals.

7 Conclusion

Currently, whilst knee replacement surgery enjoys longevity, it is accepted that functionality still is challenging. The advent of significant amounts of anterior knee pain in our patients leads us to focus more accurately on the function of the soft tissue envelope and the articular structures that makes up the third space.

It is apparent that balancing the third space may have been relatively neglected until recently and renewed focus on the soft tissue tensions, anterior offset and function of the extensor hood may lead us to adapt the surgical process to ensure the third space is not overstuffed, leading to soft tissue pain, or understuffed, leading to loss of function.

Current technology and cutting guides do not allow the surgeon to address the challenges of the third space, they do not offer the facility of planning femoral positioning in 3 dimensions, nor record the pressure or loading in the sensitive third space. Here, there is a real option for digital and sensor technology linked with Robot assisted surgery to significantly improve the outcome of knee surgery.

More recently, it has been possible to monitor and record the real time dynamic patellofemoral forces in each quadrant of the patella during surgery. By altering the patella resection it is apparent we may address abnormalities in patella femoral loading that appear to occur after total knee replacement. It is hoped this would offer a genuine way forward in addressing the 20 % of patients who feel compromised functionally after total knee replacement.

Coupling the data from the dynamic sensor assessment of the third space with AI to allow accurate planning of femoral positioning in 3 dimensions with reliable execution by robotic assisted surgery represents an exciting step change in knee surgery, not a more accurate method to conduct standard knee surgery, but an insight to what might be possible when knee replacement is planned and completed in three dimensions with relevance to soft tissue tension and pressure for the benefit of patients and their improved outcome.

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Data availability statement:

Data is available on reasonable request by contacting the corresponding author.

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Author contributions

DB – conceptualization, writing-review and editing, writing-original manuscript, visualization; AB – writing-review and editing, writing-original manuscript.

Funding

No funding, other than institutional funding was provided for this manuscript.