1 Introduction

Patellofemoral osteoarthritis (PFOA) is a condition characterized by pain, reduced physical function, and lower quality of life.^1–4 Though it is distinct from medial and lateral tibiofemoral arthritis, PFOA can occur concurrently and even lead to tricompartmental osteoarthritis.⁵ Epidemiological studies suggest that of all compartments involved in knee osteoarthritis, the patellofemoral compartment is most commonly affected, with 2 studies suggesting around 39 %–64 % of patients with knee pain having OA with patellofemoral involvement.^6,7 As such, the incidence of OA, especially PFOA, is expected to rise as our population ages and the prevalence of obesity rises.

A key factor causing degeneration and pain in the patellofemoral compartment is the eccentric load on the patella laterally.⁸ The lateral patellofemoral joint (LPFJ) consists of associated soft tissue structures, the patella, and the femur, and its anatomy plays a critical role in joint tracking, stability, and force distribution.⁹ Collectively, these soft tissue structures are termed the lateral retinaculum, which functions to stabilise the patella and prevent patellar tilt.^10,11 Important parts of the lateral retinaculum include the lateral patellofemoral ligament (LPFL) and lateral patellofemoral ligament (LPFL), as well as other soft tissue structures such as the lateral patellomeniscal ligament (LPML), the vastus lateralis obliquus, the quadriceps aponeurosis, and the iliotibial band.^9,12

Lateral retinacular tightness results in patellar tilt, subluxation, or dislocation, which causes increased pressures on the lateral patellar facet, predisposing to patellofemoral pathology, including instability, overloading, anterior knee pain, and eventual PFOA.^9,12–14 Additionally, many studies, in both clinical and autopsy settings, have reported that PFOA most frequently involves the lateral facet, with up to 89 % of isolated PFOA cases showing lateral facet involvement.^15–17 Hence, lateral-sided surgical procedures targeting the lateral retinaculum and lateral patellar facet can slow progression of PFOA and alleviate symptoms.^14,18,19

Many studies have demonstrated the effectiveness of partial lateral facetectomy (PLF) in reducing symptoms of PFOA, while preserving native knee anatomy and delaying the need for more invasive surgeries.⁹ During this procedure, a part of the lateral facet is resected, along with any osteophytes in this region, which would decompress the LPFJ and functionally lengthen the lateral retinaculum, thus reducing pain in this area that contains a high density of nerve.^9,20–22 The goal of the PLF is to relieve symptoms and maintain preoperative functional capacity, but not to eliminate predisposing factors.^23,24 Normal patellofemoral biomechanics is also shown to be preserved with 25 % removal of either medial or lateral facets,²⁵ which most studies adhere to with only 1–1.5 cm of the lateral facet being resected in PLF.^23,24,26,27

As such, the ideal candidates for PLF would be young patients who wish to maintain an active lifestyle, and have a normal valgus vector at the patellofemoral mechanism,^23,24 a population demographic which more invasive procedures such as TKA would be less suited for.^26–28

PLF is commonly associated with other procedures, most commonly lateral retinacular release or lateral retinacular lengthening, which manipulate the lateral retinaculum and release pressures on the lateral facet. Paulos et al.²⁶ theorises that the denervation of the lateral retinaculum through lateral release as well as the resection of “kissing” osteophytes from lateral patella and femur, decreases pain and improves function. The addition of the bony resection in a PLF further reduces mechanical crepitus and contact pressures on the lateral femoral condyle, eliminating an additional source of pain, leading to more favourable results in PLF associated with lateral release compared to lateral release in isolation.^26,29 Realignment procedures, either proximal or distal, have been reported to achieve good results in young patients with patellofemoral pain and instability, and have been used in some studies alongside PLF as well.²⁶

Various studies have evaluated the clinical outcomes of partial lateral patellar facetectomy on PFOA but, to the best of the authors’ knowledge, no meta-analysis has been done to evaluate these claims. This systematic review and meta-analysis thus aims to evaluate the effectiveness of these procedures and contribute to operative decision making when treating PFOA.

2 Material and methods

3 Results

3.1

3.1 Literature retrieval

The search strategy yielded 1817 articles, and no additional articles were identified through citation searching. After removal of duplicates, 1454 articles were screened and 75 full-text articles were retrieved and assessed for eligibility based on the inclusion and exclusion criteria in Section 2.2. The PRISMA 2020 flow diagram (Fig. 1) describes the exact process. In total, 10 articles were included for systematic review, while 6 articles could be meta-analysed. All studies that were included achieved a moderate to high score on the Newcastle-Ottawa Scale (Table 1), with one study achieving a score of 6, another study achieving a score of 7, and the remaining eight studies achieving a total score of 8.

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Fig. 1 PRISMA 2020 flowchart.

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3.3

3.3 Meta analysis of outcome measures

3.3.1

3.3.1 Knee Society Score (KSS)

5 studies compared total KSS scores among 150 patients. The decision was made to standardise the KSS as a sum of knee clinical and functional scores due to the variance in reported values as individual scores and total scores. The pooled mean age amongst the study populations is 58.98, and the mean follow-up time is 63.6 months. The pooled postoperative KSS was 161.60 (95 % CI: 154.42 to 168.78) (Fig. 2) and the mean difference was 34.45 (95 % CI: 26.41 to 42.49), with a significant difference between preoperative and postoperative groups (p < 0.01). (Fig. 3).

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Fig. 2 Forest plot of proportional analysis of postoperative KSS.

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Fig. 3 Forest plot of pairwise analysis between preoperative vs postoperative values of KSS.

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3.3.2

3.3.2 Congruence angle (CA)

4 studies compared congruence angles among 147 knees. The pooled mean age amongst the study populations is 58.92, and the mean follow-up time is 57.63 months. The pooled postoperative CA was 15.98 (95 % CI: 10.95 to 21.01) (Fig. 4) and the mean difference was −10.31 (95%CI: 13.80 to −6.81), with a significant difference between preoperative and postoperative groups (p < 0.01). (Fig. 5).

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Fig. 4 Forest plot of proportional analysis of postoperative CA.

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Fig. 5 Forest plot of pairwise analysis between preoperative vs postoperative values of CA.

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3.4

3.4 Subgroup analysis

Further subgroup analysis was conducted to assess the difference in outcomes based on different concomitant procedures. Procedures were split into 3 groups: “Group 1” being lateral retinacular release, “Group 2” being lateral retinacular lengthening, and “Group 3” being realignment procedures. There was a significant difference (p < 0.01) in subgroups for KSS (Fig. 6), which showed a larger improvement for the lateral lengthening group as compared to the lateral release and realignment groups. On the other hand, no significant difference (p = 0.65) is seen in the subgroups for CA (Fig. 7).

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Fig. 6 Forest plot of subgroup analysis of KSS. Group 1 = PLF + Lateral release, Group 2 = PLF + Lateral lengthening, Group 3 = PLF + Realignment.

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Fig. 7 Forest plot of subgroup analysis of SA. Group 1 = PLF + Lateral release, Group 2 = PLF + Lateral lengthening, Group 3 = PLF + Lateral release + Realignment.

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4 Discussion

Partial lateral facetectomy of the patella has been shown to be an effective and viable procedure to treat isolated lateral PFOA in younger, active patients.^{23,24,26,28,38,39} Multiple studies have touted PLF's benefits as a low-risk procedure that preserves native knee function, while having minimal effect on future knee procedures and arthroplasties.^{9,23,24,40,41} With the multitude of treatment options for isolated PFOA, however, there is still no consensus on the optimal procedure for isolated PFOA. As such, the aim of this study was to assess the effectiveness of PLF on patients with PFOA. Our meta-analysis has demonstrated that PLF, alongside concomitant lateral retinacular release, lengthening, and realignment procedures showed significant postoperative improvement in subjective and radiographic outcomes across multiple studies.

Out of all studies included in this review, only Becker et al.³⁵ did not recommend the use of their modified PLF which involved lateral release and tibial tubercle medialisation (modified Elmslie-Trillat technique), as they reported unsatisfactory results that were inferior to other procedures such as isolated facetectomy or tibial tubercle transfer. A possible reason for the underwhelming results would be the lack of tibial-tuberosity-trochlea groove (TT-TG) measurement through computed tomography (CT) scanning preoperatively, which is required for effective medialisation of the tibial tubercle.^19,35

In terms of subjective outcomes, this study showed significant improvement in KSS in our meta-analysis. KSS has been reported to be a reliable and valid outcome assessment tool,⁴² with anatomic KSS shown to be a significant predictor against failure of PLF.⁴³ Other patient reported outcome measures (PROMs) were used in other studies, but there was a lack of uniformity in the use of PROMs. There were 3 studies that reported VAS,^18,28,36 and another 3 studies that reported Kujala scores,^19,26,27 but were unable to be meta-analysed due to a lack of reported data. Other PROMs that were analysed include International Knee Documentation Committee (IKDC), Knee Osteoarthritis Outcome Score (KOOS), WOMAC, Lille, and McCarroll scores. 3 studies evaluated subjective patient satisfaction, showing 88–93.3 % in a mean follow-up of 60–90.1 months,^26–28 and 2 other studies evaluated subjective pain relief, with 78–84 % showing significant pain relief in a mean follow-up of 24.1–126.2 months.^24,36 Ultimately, all studies report significant improvement in subjective outcomes in PLF, which indicate that PLF is an effective procedure. However, more investigations need to be conducted with other well-established PROMs so as to evaluate the effect of PLF on PROMs more holistically.

In terms of radiological outcomes, this study shows improvement in congruence angle in our meta-analysis. Congruence angle is a measure of the position of the patellar in relation to the trochlear groove, and was used to valuate improvement in patellar location.^27,35,36 All studies that measured congruence angle had significant improvements, and 3 out of 4 studies had a mean CA that was considered physiologically normal (<16°).⁴⁴ Becker et al.³⁵ showed improvement, but on average still had a mean CA of 24.5° which indicated patellar subluxation.⁴⁴ Other radiographic measures of patellar instability were conducted in some studies – 3 studies showed significant postoperative decrease (p < 0.001) in patellofemoral index, a measure of thickness of the medial and lateral PF space; 3 studies evaluated the subluxation of patella: Yercan et al.²³ and Lopez-Franco et al.²⁴ had a non-significant increase in number of centralised patellas postoperatively, while Douiri et al.²⁸ showed a significant decrease (p < 0.0001) in patella subluxation; Monserrat et al.¹⁹ showed a significant decrease in patellar tilt (p = 0.001); Becker et al.³⁵ and Akilzhanov et al.³⁶ reported a non-significant decrease in sulcus angle, which connotes trochlear depth, though they remained within physiologically normal range of 138° ±6°.⁴⁴ This data is interesting to note, as the use of PLF, lateral release, and lateral lengthening has been contentious in the treatment of patellar instability, and some studies have recommended avoiding these procedures when treating PFOA secondary to patellar instability or subluxation.^24,40 While other studies mentioned above excluded patients with patellar subluxation, Yercan et al.²³ and Douiri et al.²⁸ utilised PLF with lateral release while including patients with patellar subluxation, and reported non-significant and significant decrease in patellar subluxation respectively. As such, more studies need to be done to evaluate the efficacy of this procedure on patients with PFOA with or without patellar subluxation. Additionally, studies have also measured changes in severity of osteoarthritis. 4 studies measured Iwano staging for PFOA preoperatively and postoperatively, though there were no significant changes in population. 5 studies measured TFOA involvement as measured by Ahlbäck and Kellgren-Lawrence classification, with all studies showing an increase in number of TFOA cases or progression of TFOA cases at final follow-up, though only 2 studies had a significant increase.

In terms of complications, 6 studies reported complications, with 4 of these studies having conducted a long-term survival analysis, and the remaining 4 studies reported no complications postoperatively. Out of the 6 studies, various postoperative complications were described, such as hemarthrosis, prolonged effusion, mild complex regional pain syndrome, transient extension lag, internal saphenous neuroma, though they constituted a small number of patients in the study population and were not significant.^28,37 More significantly were the evaluation of failure rates of the procedure as noted by the Kaplan-Meier survival curves of the study populations. Failure is defined as the need for additional revision surgeries to the patella, including total knee arthroplasty (TKA), patellofemoral arthroplasty (PFA), and patellectomy.^19,24,37 Paulos et al.,²⁶ Wetzel et al.,³⁷ Lopez-Franco et al.,²⁴ Montserrat et al.,⁴³ Douiri et al.²⁸ noted failure rates of 17 % in 60 months, and 36.9 %, 32.35 %, 26.5 %, 16 % respectively in a range of mean follow-up timings from 5 to 11 years. Wetzel et al.,³⁷ Montserrat et al.,⁴³ Douiri et al.²⁸ also reported extremely positive long-term results, with all 3 studies reporting cumulative survival rates of 85.0 %–96.4 % at 5 years in study populations of 168, 43, 56 respectively, and Wetzel et al.³⁷ and Montserrat et al.,⁴³ further noted 55 %–59.3 % survival at 15 years. With low to moderate failure rates and high survival rates in the long term, PLF is shown to be a good procedure that also effectively delays the need for arthroplasty procedures.

TKA and PFA have been reported to be highly effective in treating PFOA – with TKA remaining the most proven surgical option for older PFOA patients, and PFA as an alternative for patients younger than 55.^{24,38,45–48} The use of TKA, a tri-compartmental procedure, to treat an uni-compartmental issue has been debated and deemed less suitable for a younger, more active demographic,^{24,26–28,39,49} while PFA's success rates of 44–90 % and revision rates of 20–40 % at mid-to long-term follow up are lower than that of PLF.^50,51 As such, Lopez-Franco et al.²⁴ advocates for the use of PLF over PFA and TKA, citing its lower cumulative failure rate and easier conversion to TKA. Similarly, Douiri et al.²⁸ supports PLF as an option to delay TKA and PFA due to its minimal invasiveness, low complication rates, and ease of conversion to arthroplasty.

Our subgroup analysis was intended to highlight potential differences in procedures concomitant to PLF. Significantly, we found that there may be a potential benefit of lateral retinacular lengthening over release and realignment as seen in significant improvement of KSS in Wang et al.,²⁷ compared to the other 4 studies (Fig. 6). This is supported by a prospective study by Pagenstert et al.⁵² which demonstrated better outcomes at 2 years for lateral lengthening than release in patients with lateral patellar pain. Other studies such as Wetzels et al.³⁷ found no significant difference between patients undergoing isolated PLF and PLF with lateral release. Various realignment procedures have been shown to aid in patellar instability,¹⁹ and have achieved good results in young patients with patellofemoral pain and instability, but the efficacy is less clear in patients with lateral facet PFOA and older patients.^26,53,54 Due to the lack of literature, more studies would need to be conducted to adequately distinguish the effect of these concomitant procedures on PLF efficacy. However, this is the first attempt, to our knowledge, to dichotomise whether different concomitant procedures had any difference in outcomes, and should act as a springboard in future investigation into surgical treatment of PFOA.

There were some limitations to our review. Firstly, there were no randomised clinical trials found comparing facetectomy with other forms of treatment in the context of PFOA, and as such, most studies included were retrospective in nature. Secondly, many of the subjective outcome measures were not adequately reported among all papers which made comparison of clinical outcomes more challenging. Thirdly, the realignment techniques included in our meta-analyses had significant realignment that could be a confounder to our results. However, we still opted to use these studies to assess how they would perform to other studies without realignment. Fourthly, there is heterogeneity to the procedures as part of the analysis. However, as this is the first paper, to our knowledge, that reviews and meta-analyses the clinical outcomes of partial lateral facetectomy in patients with PFOA, our conclusion that there is significant clinical benefit provides valuable insight to the use of this procedure in the context of PFOA.

5 Conclusion

In conclusion, this study highlights PLF as a viable surgical treatment option when treating isolated PFOA in the younger population, due to its simplicity, low risk, and ability to preserve natural knee kinematics. To our knowledge, this is the first systematic review and meta-analysis that highlights the clinical outcomes of PLF on patients with PFOA, demonstrating significant improvement in subjective and radiographic outcomes, as well as low to moderate long-term failure rates and high survival rates up to 15 years. This study adds valuable insight into the use of PLF as surgical options in addition to other options such as lateral retinacular release and realignment procedures.

CRediT authorship contribution statement

Kennan ZG. Yeo: Conceptualization, Methodology, Investigation, Formal analysis, Visualization, Data curation, Writing – original draft. Mark HX. Yeo: Methodology, Investigation, Validation, Formal analysis, Data curation. Shawn JS. Seah: Methodology, Investigation, Validation, Formal analysis, Data curation. Winston SR. Lim: Validation, Supervision. Denny TT. Lie: Conceptualization, Supervision.

Informed consent

Not Applicable.

Guardian/patient's consent

Not Applicable.

Ethical statement

This was an IRB exempt study.

Funding statement

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. The authors have no relevant financial or non-financial interests, and no competing, financial or proprietary interests in any material discussed in this article. All authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this manuscript.