By: 22 January 2018
Patient-specific instrumentation for total hip and knee arthroplasty: Is it the new surgeons’ guide to improve implant positioning?

Robert Wescott, Johann Henckel, Anna Di Laura and Alister Hart discuss the factors associated with the clinical outcome and longevity of knee and hip implants



The clinical outcome and longevity of knee and hip implants is associated with surgeon, implant and patient factors. In both total hip arthroplasty (THA) and total knee arthroplasty (TKA), the implant position contributes to the survival and clinical functionality of the implant [1,2].

The position of the femoral and the acetabular components in a THA affect patient satisfaction [3], as this position determines the final leg length and the range of motion within the hip joint [4,5]. Mal-positioned THA components have demonstrated increased rates of wear, impingement and dislocation [6,7]. Poor placement of the tibial component in TKAs has been associated with loosening [8], therefore, one way to improve the lifespan of prosthesis is to implant them into the “correct” position.

Over the last decade, computer-assisted navigation and robotic systems have been introduced to aid in the intraoperative positioning of implants. Robotic systems are either active or semi-active and require a pre-operative CT scan to create a map of the patient’s anatomy. Active systems direct the surgeon’s instrument placement while semi-active surgical robots create pre-defined boundaries for bony preparation [9]. Although these systems have demonstrated an increased accuracy in component positioning for THA, they have a high initial cost and they increase the total operative time [10-12].

Navigation systems are always passive. They work by visually guiding the surgeon in to aid their operative technique. CT-based navigation systems have demonstrated good long-term clinical results for THAs, however they have issues due to the increased exposure to radiation. Other navigation systems use fluoroscopic imaging, however they have had a low acceptance due to concerns over their accuracy [13].

Figure 1: This image depicts the process of producing a 3D computer generated model from CT imaging to allow for the planning of the surgical procedure and the production of PSI jigs. A) Thin CT slice through the pelvis and upper femur, B) thin CT slice through the distal femur and C) the final surgical plan produced using the CT data.


Figure 2: A) A 3D model of the acetabulum is printed to aid the surgeon in placing the acetabular guide. B) The custom acetabular guide sits against specific bony landmarks within the acetabulum. This guide is used to insert two guide wires into the acetabulum. C) A visual guidance bar is then slid over the two wires, to aid the surgeon in visualising the reaming and final implant trajectory. D) A 3D printed model of the right hemi-pelvis demonstrates the optimal position of the new cup.



Patient-specific instruments (PSI) are an alternative to both navigation and robotic surgical guidance for implant positioning. PSI systems are custom made on a case-by-case basis, specific to both the anatomy of the patient and the surgical plan made by the surgeon. Each patient requires a low-dose pre-operative CT scan of the surgical site. Computer software is then used to generate a virtual 3D model of the patient’s bony anatomy [14]. These virtual models are used by the surgeons to aid in planning the position, size and orientation of the implant relative to a defined frame of reference, for example, the mechanical axis of the lower limb in knee arthroplasty or in the hip in the anterior pelvic plane (APP). Commercial PSI engineers then work with the surgeons to design the PSI to aid in the intraoperative execution of the surgical plan. Rapid prototyping (RP) is used to “3D print” the PSI before it is sterilised and sent to the surgeon.


Patient-specific instruments for total hip arthroplasty

Current solutions

The ultimate goal of PSI when used for THA is to enable the surgeon to deliver their surgical plan and achieve a greater accuracy in the positioning of both the femoral and the acetabular components [15-17].

Acetabular PSI systems are supplied by at least four commercial companies (see table 1). These guides (both physical and laser) are used to optimise the cup size, medialisation and orientation (anteversion and inclination). There are two forms of acetabular guide: constrained or non-constrained. Constrained guides physically constrain the trajectory of both reaming and implant positioning. Non-constrained PSI guides act as a visual representation of the cup trajectory for the surgeon to follow (see figure 2).

Femoral PSI systems are produced by two commercial companies (see table 1) to assist in the femoral cut. They are used to optimise the stem size, position, offset, version and leg length of the patient. These are small block jigs, which are designed dependent on the surgical approach. These fit to the anatomy of the femoral head and neck to guide the angle and position of the femoral neck cut (see figure 3).

Patients undergo a low-dose CT scan of the pelvis and the hip of interest taking 11 minutes and costing $55 [14]. The scan is used to create a virtual 3D model of the hemi-pelvis and proximal femur, which is used to plan the size and position of the implant (see figure 1). My Hip (Medacta, Castel San Pietro, Switzerland) and OPS (Corin, Cirecester, England) use kinematic simulation to assess the functional impact of the implant in terms of both bony and implant impingements. The PSI guides and jigs are then manufactured with either selective laser sintering or additive materials manufacturing (both forms of RP). This process can take as little as three weeks [18] but typically has a lead time of six to eight weeks.

Alongside the PSI guides, physical 3D models of the hemi-pelvis and proximal femur (when femoral guides are used) are produced, sterilised and used intraoperatively (see figures 2a and 2d). These allow for visualisation of the patient’s own bony anatomy, demonstrating in 3D space how the guide will fit prior to use on the patient. These additionally guide in the process of osteophyte removal and soft tissue dissection; if the anatomy at the contact points between the PSI guide and the bone is disrupted from the original CT scan, the guide is no longer representative of the plan [19].

Figure 4 demonstrates a primary total hip replacement case completed at our centre using hip PSI.

Figure 3: A) The femoral neck cutting jig is designed to press against the patient’s unique bony anatomy. This is then pinned in place. B) The jig acts as a guide for cutting through the femoral neck, setting both the height and the angle of the cut. C) The femoral head is removed with the jig attached.



Acetabular PSI and its influence on cup orientation

The component position effects the clinical outcome of a THA [2], so short-term data with acetabular PSI has focused on planned vs. achieved cup orientation.

In a prospective randomised controlled trial, Small et al. compared planned vs achieved data for 18 patients who underwent PSI guided THAs with 18 patients who underwent standard THAs [20]. They looked at the difference between planned and achieved inclination and anteversion. The operations that utilised PSI guides achieved a significantly lower discrepancy in planned vs achevied anteversion angle with a mean difference of 0.2˚ ± 6.9˚ compared to 6.9˚ ± 8.9˚ for the standard procedure. The planned vs achieved difference between the PSI and standard group was insignificant for cup inclination.

Hananouchi et al. have demonstrated that PSI for acetabular implantation reduces the percentage of malpositioned implants out of the “safe zone” of 15˚ ± 10˚ for anteversion and 40˚ ± 10˚ for inclination when compared to standard (0 per cent compared to 23.7 per cent) [16]. Spencer-Gardner et al. demonstrated that 91 per cent of operations using the Corin acetabular system were within 10˚ of their plan for both version and inclination, which was superior to free hand surgery [18].


Femoral PSI and its influence on stem orientation

Femoral PSI is less common, so the literature is limited in this field. Ito et al. looked at femoral stem placement using a femoral PSI solution [21]. Although this wasn’t a comparative study, they reported a mean accuracy of stem tilt as 2.1˚ ± 4.1˚, varus/valgus as 1.0˚ ± 0.7˚ and anterversion as 4.7˚ ± 1.2˚. Further work is required to assess how femoral PSI solutions affect leg length discrepancy and final implant functionality.


PSI in THA and its influence on surgical time

One study demonstrated that the mean time spent using the PSI surgical guides intraoperatively was 3.6 minutes [16]. A navigated THA can add between five to 58 minutes onto the operative time, whereas the use of an acetabular PSI adds between three to seven minutes onto the operation [18,20].


The cost of implementing PSI for THA

As demonstrated, there are four commercial PSI systems, with an average associated cost of $371 per case. It is not yet known if the cost of the PSI is economically advantages. Long-term implant survival studies are needed to assess if better implant positioning achieved by PSI leads to better implant survival and reduced rates of revision.


Intraoperative complications with PSI

In the literature reviewed for this article, there were no additional complications associated with PSI for THA. There was no significant difference in blood loss when PSI was used compared to standard instruments [16,20]. There are currently no long-term studies to look at implant survival.


Patient-specific instruments for total knee arthroplasty

Current solutions

The main goal of PSI for use in TKA is to increase the accuracy of the bony cuts made prior to implantation in order to improve the final position of the tibial and femoral components with the goal of maximising implant survival. TKAs need to be accurately placed to recreate the mechanical axis of the limb. Higher complication and increased in polyethylene wear rates have been reported when this is not achieved [22].

A selection of TKA PSI solutions [23] are shown in Table 2. The majority require a pre-operative CT or MRI scan of the knee, with slices through the ankle and hip so the mechanical axis of the knee can be derived (necessary for planning).

Additionally, in Europe, a commercial PSI system for TKAs using solely plain film X-rays has been introduced [24]. Virtual 3D models of the tibial plateau and femoral condyles are created from the imaging data and used for planning to determine the implant size and orientation [25]. The jigs are designed to match the bony contours of this virtual model to allow for a precise fit between the PSI guide and the patient’s anatomy. These are manufactured using RP, along with models of the patient’s bony anatomy, and then sterilised before being sent to the surgeon (see figure 5).

The majority of commercial knee PSI systems aim to reconstruct the knee to the computed mechanical axis, however, the OTIS system reconstructs the knee back to the pre-arthritic anatomical shape. Studies have demonstrated that the anatomical alignment of the knee varies greatly from the mechanical axis, and the two are not always identical [26].

There are two types of knee PSI systems. Pinning TKA PSI systems guide the placement of pins for use with commercial cutting guides. These sit over the patient’s anatomy and act as drill guides. Cutting TKA PSI guides are sat against the patient’s anatomy, pinned onto the femoral condyles and tibial plateau and are used as a cutting guide [23].

Figure 5: The DePuy Synthes TruMatch TKA PSI system. A-B) show the planning document produced from CT patient data. This is used to plan the position of the implant and design the cutting jigs. C-D) shows the femoral cutting jig. E) shows the tibial cutting jig.


TKA PSI and its influence on alignment

Knee PSI systems have been demonstrated to improve the alignment of the knee post–TKA closer to the presumed normal mechanical axis in comparison to conventional jigs [27-29]. Anderl et al. demonstrated a less than 2˚ deviation from the planned component positions using PSI in TKAs. They showed a significantly superior accuracy in mechanical alignment restoration compared to conventional instrumentation [30].

Roh et al. demonstrated PSI systems to be non-advantageous over conventional systems, with no significant difference in hip-knee-ankle angle outliers between the two groups [31]. Additionally, they demonstrated that in 16 per cent of TKA cases, the PSI guides were abandoned due to malrotation of the femoral components and a decreased slope of the tibia. Other studies have demonstrated that PSI used in TKA decreases the accuracy for the tibial slope compared to traditional instruments [32].

The benefits of PSI in TKA have not been universally demonstrated in the recent literature. In our practice, we use nets of protection to ensure the correct position of the implant is achieved. As well as using PSI guides to aid surgical technique, we ensure that the position is agreed with the surgeon’s own clinical experience.

The literature is divided on the benefits of reconstructing the knee back to the pre-arthritis anatomical shape using the OTIS Knee PSI system. Two studies have demonstrated that this system leads to an accurate reconstruction of the mechanical axis in the knee [33,34], however one paper has demonstrated poor outcomes [35]. The tibial and femoral components, using the anatomical reconstruction PSI system, were placed out of the accepted range of alignment.


TKA PSI and its influence on surgical efficiency

The use of PSI for TKA in a randomised trial demonstrated a reduction in the surgical time taken to complete the operation when compared to a standard TKA (121.4 minutes to 128.1 minutes). Additionally, the operation that utilised the PSI required a reduced number of instrument trays (4.3 compared to 7.5) [27].

TKA PSI and its influence on functional outcomes

There is little literature on the effect of PSI guided surgery on the functional outcome for the patient. It has been demonstrated that functional scores are superior in patients who have their knee reconstructed to within 3˚ of the presumed normal mechanical axis [1].

It is yet unclear if PSI used in TKA provides an advantage for component positioning, so long-term data is required to see how PSI guided implants survive.


TKA PSI and its cost effectiveness

Like PSI for THA, it is impossible to know if the economic burden of producing PSI guides will be offset by longer lasting implants. Long-term data is once again needed to determine if PSI systems are economically viable.


Rapid prototyping of PSI guides for both TKA and THA is still in its infancy. Short-term data for THA PSI suggests these systems are improving component positioning, allowing surgeons to fulfil their pre-operative plans with a higher level of accuracy. The data is more inconclusive when comparing TKA PSI to standard instrumentation. Unlike navigation and robotic solutions, PSI systems do not add a significant amount of time to the procedure and come at a much lower initial cost. It is yet to be seen if improving surgical accuracy and correctly achieving the pre-operative surgical plan using PSI will develop into the improved survival of knee and hip implants.




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