Masticatory Efficiency of Mandibulectomy Patients after Oral Prosthetic Rehabilitation: Preliminary Results

Objectives: This case series pilot study aimed to test the masticatory efficiency of mandibulectomy patients reconstructed with the free fibula flap using a modified mixing ability test and a digital colorimetric assessment.

Material and Methods: Ten healthy dentate volunteers chewed a two-colour chewing gum for 30, 40, and 50 cycles to determine the minimum number of chewing cycles required to achieve consistently at least 90% colour blending (50 cycles). Mandibulectomy patients (n = 6) were given the same chewing gum and separate specimens were collected from the operated and non-operated side after (50 chewing cycles). The masticatory efficiency was assessed by: 1) photometric analysis to calculate the percentage of blended surface; and 2) measuring the colour blending deviation (ΔE) compared to healthy volunteers’ master specimen (P < 0.05).

Results: The test group of cancer patients registered a mean 76.75% surface with blended colour after 50 chewing cycles at the non-operated site, and at the operated site (fibula free flap and implant-supported fixed prosthesis) a mean 60.08% blended colour surface was found. The colorimetric analysis revealed significant differences in ΔE values between all the investigated groups (cancer patients operated site < cancer patients non-operated site < healthy patients, P < 0.001).

Conclusions: The qualitative comparison of masticatory efficiency between test and control group of healthy patients showed different values of efficiency, attesting lower masticatory performance for mandibulectomy patients, especially in the operated site of small extent rehabilitation.

J Oral Maxillofac Res 2025;16(3):e5

doi: 10.5037/jomr.2025.16305

Accepted for publication: 27 September 2025

Keywords: CAD-CAM; dental prosthesis design; dimensional measurement accuracy; fibula; mandibulectomy; masticatory force.

INTRODUCTION

The gold standard for the surgical treatment of mandibular oral cancer involves the use of the free vascularized fibula flap supported by a custom-made titanium plate, functionally rehabilitated with an oral implant-supported fixed prosthesis [1]. While new digital protocols have been proposed over the past two decades, masticatory function rehabilitation remains a challenge [2]. The prosthetically guided maxillofacial and oral surgery (PGMS) protocol was introduced as a viable solution to the prosthetic challenges of functional and masticatory rehabilitation after cancer surgery [3,4]. Indeed, the PGMS allows preventing the post-surgical facial disfigurement by recovering the native anatomy of the mandible contour; and moreover, it allows positioning the free fibula flap at the level of the alveolar bone of the mandible, and placement of implant-supported restorations for the optimal masticatory function of the prosthetic rehabilitation. Recent study by Zhu et al. [5] has investigated the masticatory performance of patients who received dental rehabilitation after mandibular reconstruction with a free fibula flap, determining differences between fixed, removable, or implant-retained removable prosthetic rehabilitation. Zhu et al. [5] concluded that the dental implant prosthesis had a more positive effect on the masticatory performance. Despite the excellent results achievable with this reconstructive digital approach after cancer removal, patients often experience a subjective perception of reduced chewing ability [5]. Scientifically, this feature is well-defined as masticatory efficiency, which is the “effort required to achieve a standard degree of comminution” [6,7]. A decrease in masticatory efficiency in patients who underwent jaw reconstruction with a free fibula flap can lead to a decline in tongue motor functionality, perioral muscle function, and occlusal force [8]. This progressive deterioration in masticatory performance may further lead to a condition known as “oral frailty,” which can result in impaired self-perception and quality of life, and dietary deficiencies [9,10-12]. The knowledge gap between the surgical results of the PGMS and its recovery of the masticatory performance compared to healthy individuals has not yet been investigated.

Various methods have been proposed to objectively assess masticatory performance, including evaluating the particle size distribution of natural or synthetic foods after a specific number of chewing cycles. Natural foods such as carrots, coffee beans, peanuts, almonds, squid, or bread [13-17], as well as synthetic ones like silicone or bicolour chewing gum [18-20], have been used. While tests with natural foods present challenges in terms of fragment selection, spitting out and rinsing, stacking into a sieve, drying, and weighing, synthetic foods offer practical advantages. Several groups of authors [20,21] used a two-colour chewing gum to correlate the degree of colours blending to the masticatory performance and to the number of chewing cycles [22-24]. However, there is limited research comparing different diagnostic methodologies to measure masticatory performance and proposed the colour-changing gum test, and the mixing ability test with visual or electronic analysis as useful screening alternatives to the gold standard Jelly-scan test [25]. Similar methodologies have already been established by other research groups. Similarly, more recent research by Homsi et al. [26] evaluated masticatory function in older individuals using bimaxillary implant-supported prostheses, providing valuable insights into functional outcomes with natural and prosthetic dentition.

Accordingly, this pilot study aimed to employ a modified mixing ability test described by Schimmel et al. [24] and a novel digital colorimetric assessment to test masticatory performance of mandibulectomy patients to determine the clinical efficacy of PGMS rehabilitation method compared to healthy individuals. Moreover, for each operated patient the healthy site was compared to the rehabilitated, to explore the asymmetry and quantify the recovery of the masticatory function after surgery of both sites. The proposed methodology aimed to test the primary null hypothesis that no difference exists between patients who underwent mandibular reconstruction with a free fibula flap and healthy participants. Additionally, this case series pilot study aimed to test the secondary null hypothesis that no difference is present between the masticatory performance of the operated and native sites in the same patients who underwent surgery.

MATERIAL AND METHODS

The study was conducted at the Maxillofacial Surgery Section of the Department of Biomedical and Neuromotor Science, University of Bologna, Bologna, Italy since January 1, to July 31, 2023.

The PGMS protocol of this study was approved by the Ethics Committee of S. Orsola University Hospital, Bologna, Italy (No. 57/2011/O/Disp). All participants have read and signed informed consent form.

Briefly, the PGMS aims to project the maxillofacial and implant surgeries guided by the native position and anatomy of teeth, that should guide the final design of the implant supported fixed prosthesis: the position of the free fibula flap and the oral implant position in the microvascularized transplanted bone is driven by the prosthetic project.

Subject, inclusion and exclusion criteria

Two subject samples were created for this pilot study to test the metrological methodology: the test group (n = 6), representing cancer patients rehabilitated with the PGMS protocol and fixed implant-supported prosthesis (Figure 1) and the control group (n = 10) representing healthy fully dentate patients.

Inclusion criteria for cancer patients included no age limit, mandibular reconstruction with a free vascularized fibula flap since more than 12 months, no facial pain, and no impairment of temporomandibular joint (TMJ) movements. Inclusion criteria for the fully dentate group included angle class I, Decayed, Missing, and Filled Permanent Teeth (DMFT) index < 4, and an age between 18 and 40 years.

Exclusion criteria for both control and test groups included diabetes-related pathologies, presence of post-surgical scar bands limiting masticatory movements, and neuromuscular disorders. The test group was represented by 5 male and 1 female patients (Table 1), with 39 years mean age (range 19 to 61), that underwent mandibular reconstructive surgery with free fibula flap due to different oncologic diagnosis. Depending on the anatomic mandibular area of cancer involvement and by the benign/malignant diagnosis, the extent of the bone defect was different for each patient, classified with the Tarsitano [27] scale classification that proposed a reconstructive algorithm considering the different types of defects of the mandible. The study proposed three classes of defect plus a subclass Ic, depending on the surgical involvement of the body, the ramus, the condyle and/or the symphysis of the mandible [27].

Specimens

Two-colour chewing gum (Big-Babol - Perfetti Van Melle; Milan, Italia) was selected for this study due to its double-colour composition comprising pink and white parts, making it suitable for colorimetric evaluation.

Each commercial gum was divided into two symmetric bicolour parts (Figure 2A) measuring 12 x 12 x 18 mm and weighing 3.5 grams using a surgical blade. The specimens were evaluated at macroscopic level by a photometric digital process and at millimetric level by a colorimetric approach.

Photometric analysis

Ten healthy dentate volunteers (control group) chewed the gum for 30, 40, and 50 chewing cycles to determine the minimum number of cycles required to achieve at least 90% colour blending. Before mastication began, the gum was manually placed by the patient on the occlusal molar surface, with the white part facing the cheek and the purple part toward the tongue. After each chewing sequence, the patient paused for 3 minutes to relax masticatory muscles and TMJ ligaments. The specimens were removed from the mouth, placed between two microscopic glass slides (Figure 2B), and flattened using manual pressure to achieve a 2 mm thickness (Figure 2C), confirmed using an electronic caliper (Fisherbrand™. Traceable™ - Thermo Fisher Scientific Inc.; Waltham, Massachusetts, USA). The specimen thickness of 2 mm was arbitrarily chosen as to obtain a standardized specimen size and to expose a larger surface of the specimen to the analysis. The pressure applied to the chewing gum offered a flat surface for analysis and prevented potential artifacts from appearing, such as saliva and large bubbles. Smaller bubbles or creases in the specimen, as well as points in the most outer angles of the specimen being overexposed to the camera flashlight were not always possible to avoid.

The specimens were photographed using DX-format digital camera - Nikon D7200 (Nikon Co.; Tokyo, Japan) equipped with a Nikkor AF-S 24-85 mm f/3.5-4.5G ED VR lens (Nikon Co.; Tokyo, Japan). The photographs were taken always in the same room at daylight (between 12 and 14 h) using the same flashlight. The JPEG files were uploaded into the open-source software - ImageJ^® version 2.9 (Fiji distribution; National Institute of Health, Bethesda, Meryland, USA). In this study, the analysis performed with ImageJ^® was photometric in the sense of colour-based image analysis, not brightness/intensity measurements using the HSB colour space. The photographs of the chewed gum samples were processed in ImageJ^® by applying a colour threshold to distinguish blended (mixed-colour) areas from unblended (distinct-colour) areas. The software quantified the proportion of the gum surface where the original colours had blended. This approach captures the spatial distribution of colours within the sample rather than measuring absolute light intensity.

The colour threshold was adjusted to a value of 90 to 130 pixels using the “Color Threshold” tool, identifying all areas of blended colour while excluding completely white or purple areas (Figure 2D, 2E). The “Analyze Particles” tool was used to obtain the percentage value of the blended surface. This constitutes a colour-based photometric analysis capturing spatial colour distribution. Once the chewing cycle number that consistently resulted in > 90% blended surface was identified, this cycle number was used for all subsequent evaluations (with both healthy and operated patients). The test group chewed on both the operated and non-operated sides of the mouth to compare mastication performance in these regions and with the control group.

Colorimetric analysis

The specimens used for the colorimetric analysis were further processed for a colorimetric assessment. One glass slide was removed, and using an 8 mm diameter circular blade, five different punches were harvested (Figure 3A, 3B), with each positioned on a thin glass slide (microscope glass cover). To eliminate bubbles in the specimens that might interfere with colour measurement, another glass cover was placed on top of each specimen, and pressure was applied to reduce the thickness of the glass slides to 1 mm and eliminate bubbles (Figure 3C). Specimens were further analysed using a live-video spot colorimeter (Easy_Color - SmartVision S.r.l.; Udine, IT) with a software capable of quantifying hue differences between the baseline blended colour (> 90%) and the experimental specimens: the colorimeter measures this difference as delta E (ΔE), that is the absorption of specific wavelengths when a light beam struck the coloured surface of the specimen. The lower ΔE value indicated better colour blending, and thus, higher masticatory performance. The colour of each specimen was measured in 3 different areas (1.5 × 1.5 mm).

The measurements were done by the same investigator (T.M.). Since the analysis was performed using a colorimeter with an inbuilt computer, the viewing conditions are set automatically and in any case do not influence the results.

Statistical analysis

For millimetric measurements by colorimetry, the ΔE values were used for group comparisons. Since the data were not normally distributed (Shapiro-Wilk test, P < 0.05), non-parametric tests (Kruskal-Wallis test and Dunn’s post-hoc test) were employed to compare the masticatory performance of healthy participants to that of cancer patients, both the operated site and the healthy side. The statistical analysis was conducted using the software SigmaPlot version 14.0 (Systat Software Inc.; California, USA). The significance level was defined at P = 0.05.

RESULTS

A preliminary macroscopic analysis determined that a minimum of 50 masticatory cycles was required to achieve a > 90% blended colour surface in specimens in healthy individuals (Figure 4). This threshold was empirically selected as the minimum to consistently achieve > 90% blending due to limitations caused by image artifacts, salivary/water residuals, and bubbles that prevented achieving a 100% blended colour surface. Photometric analysis showed different percentages of blended colour surface in specimens of ten healthy volunteers after 50 masticatory cycles (range 90 to 96%; mean 94%). Moreover, the test was conducted on specimens from the six cancer patients, with the non-operated site registering a mean 77% surface with blended colour after 50 chewing cycles (range 46 to 94%), and the operated site (fibula free flap and implant-supported fixed prosthesis, 2 to 10 prosthetic teeth) showing a mean 60% blended colour surface (range 18% to 94%) (Table 2).

Regarding the colorimetric analysis, ΔE values in cancer patients at the operated site were found to be statistically significantly higher both from the non-operated site of the patient (P < 0.001) and the healthy participants (P < 0.001). Furthermore, even at the non-operated site of cancer patients, ΔE values were found to be significantly higher than in healthy patients (P < 0.001) (Table 3).

A qualitative analysis of the results of this preliminary study shows that the different extent of the mandibular defect rehabilitated with a free fibula flap and the number of implant-supported prosthetic teeth may influence the masticatory efficiency. Table 4 displays all raw data of the test group, from the smallest mandibular defect (patients no. 1, 2 and 4) to the wider (patients no. 3, 5 and 6).

DISCUSSION

The aim of this study was to evaluate if a decrement of masticatory efficiency is present after the maxillofacial surgery of patients who underwent to mandibular resection for cancer removal and were reconstructed with a vascularized free fibula flap. The first null hypothesis that no difference exists between operated and non-operated patients, was rejected.

Also, the secondary null hypothesis that no difference of masticatory efficiency exist between the operated and non-operated site of the same patient, was rejected.

In the past, studies have reported a high percentage of unsatisfied patients rehabilitated with a removable prosthesis after undergoing mandibulectomy and reconstruction with a free-fibula flap [28,29]. Beyond aesthetics, instability, and pain, patients have reported impaired masticatory function as a primary discomfort of removable prosthetic rehabilitation [30-34]. Other studies have documented improvements in masticatory performance when an implant-supported fixed prosthesis was used to rehabilitate the masticatory function of mandibulectomy patients after bony reconstruction with a free-fibula flap [35,36]. Additionally, a positive impact on psychological well-being has been attributed to the perfect stability of the fixed prosthesis during function [37-41]. Recent studies have shown that implant-supported fixed prostheses in the reconstructed mandible have improved implant success rates and quality of life outcomes for patients [42-45].

A previous study of Tang et al. [46] has reported satisfactory masticatory performance after rehabilitation with removable or implant-supported fixed prostheses, allowing patients to have an unrestricted diet. In the same study, the authors compared the masticatory performance of mandibulectomy patients who were rehabilitated with implant-supported fixed prostheses to those who used removable prostheses or received no rehabilitation at all. They found better outcomes in the group with fixed prostheses compared to the group with removable prostheses. The patients with no rehabilitation showed the lowest masticatory performance, which often led them to adopt a liquid diet or tube feeding. However, these studies have focused primarily on dentate patients, and, apart of that study, limited research has been conducted on mandibulectomy cancer patients, in particular those reconstructed with a free fibula flap supported by a custom-made bone plate, as recently described by Tarsitano et al. [47].

In this pilot study a double metrological protocol to compare the masticatory performance of mandibulectomy patients reconstructed using the PGMS approach with that of healthy patients was developed. Two measurement methods were employed in this novel approach: colorimetric and photometric methods, both assessing the blending of two colours in a commercial bicolour chewing gum. While similar measurement methodologies have been described previously [48-50], this study focused for the first time, at our best knowledge, on mandibulectomy cancer patients and the utilization of digital image analysis to assess significant differences of colour blending. The digital processing method presented in this pilot study appears to be a reliable instrument for measuring colour blending in the two-colour chewing gum and assessing masticatory performance. Additionally, the colorimeter-based analysis eliminates inter-examiner variability and provides standardized numerical data suitable for statistical analysis. In particular, the colorimeter employed in the present study is the latest-generation instrument, initially intended for use in quality control in the optical industry and adapted for research in dentistry. Briefly, the photometric assessment of measurements consisted of the software - ImageJ^® version 2.9 (Fiji distribution; National Institute of Health) that identifies all areas of blended colour while excluding completely white or purple areas and quantify the percentage of the blended surface. On the other hand, the colorimetric assessment utilized a novel colorimeter (Easy_Color - SmartVision S.r.l.) that quantifies as Δ hue differences between two photometric images: the baseline blended colour image of the bicolour gum of the control group (healthy patients) and the bicolour gum of the test group.

The differences in the colour [49-51] are linear with the chewing functionality, since visible and measurable differences in colour mixing indicate that the chewing efficacy was not high enough to obtain the complete mixing of the 2 colours of the chewing gum. The results of both tests of the present study complemented each other and revealed significant differences in mixing ability between the control group patients, the operated and native sites of cancer patients, requiring the rejection of both null hypotheses. The digital colorimetric test demonstrated significant differences in colour of the chewing gum in cancer patients compared to the perfectly blended 2-color chewing gum (master specimen). The values of ΔE found in cancer patients were above the perceptibility threshold (Δ > 1.7) and can be detected by human bare eye. These results were in accordance with the outcomes of the photometric analysis. Among the 10 healthy patients of the control group, a minimum mixing percentage area of 90% was recorded and a maximum percentage of 96%. In the test group, on the other hand, a much wider range of variation in the mixed area among patients was present, especially regarding the operated side of the patient. In particular, at the native side of the mandibulectomy patients a smaller percentage range (variability in the outcome of the test) was found than at the operated side, but still higher than in the control group, with a colour mixing areas range between 46 to 94%. In the prosthetic (operated) side a minimum percentage of 18% and a maximum of 94% was detected, probably due to how long the patient has been wearing the prosthesis, or due to the number of prosthetic elements involved in the rehabilitation. Further studies will be necessary to split every clinical variable in single and more numerous subgroups of mandibulectomy patients, to relate the masticatory performance to the mandibular defect extent.

However, a qualitative data analysis of this study revealed that no direct proportion may be hypothesized. This qualitative analysis showed that the size of the surgically ablated mandibular area did not affect the patient›s masticatory performance in two cases. Actually, the patients with the highest number of prosthetic teeth (n = 10) were the ones with the best masticatory balance on both sides with percentages equal to or greater than 90%. Also in the colorimeter analysis, they were the patients with the lowest ΔE in the study group. No correlation may be calculated due to the small number of patients participating in this preliminary study, but a decrement of the masticatory efficiency may be noted in patients with 2 to 8 prosthetic teeth (when increasing the number of rehabilitated teeth), even if the highest values of masticatory efficiency are registered for patients rehabilitated with an almost full-arch (ten-units) implant-supported fixed prosthesis, in which a uniformly balanced occlusion resulted (Table 4). Although no definitive correlation can be established between the extent of the defect and the improvement in masticatory efficiency due to the limited sample size of this pilot study, patients no. 3 and no. 5 exhibited optimal masticatory efficiency on the operated side, as measured by both colorimetric and photometric methods. In contrast, patient no. 6 did not demonstrate such improvement. Notably, patients no. 1, no. 2, and no. 4, who presented with smaller mandibular defects, recorded the lowest masticatory efficiency values on the operated side across both measurement techniques (Table 4).

Further studies will be necessary to consider important variables in the interpretation of the data: the chewing time for performing the fifty masticatory cycles of the protocol, the size of the defect and the follow-up period before testing. During this experiment it was noted that both patients who have completed the implant-prosthetic rehabilitation since at least 2 years, were the more rapid to execute the test, probably due to the adaptation to the new neuromuscular motor patterns that allowed them getting used to the prosthesis.

In the analysis of the mandibulectomy patients the percentage difference of colorimetric analysis was not excessively large. Indeed, all ten patients of the control group obtained a mixing percentage equal to or greater than 90%, while in the study group only three out of six obtained that percentage. In the results of the test group, the value of ΔE resulted greater than the corresponding value on the healthy side without surgery, except the case of one patient in the study, for which a minimal difference of ΔE +0.42 on the healthy side was registered.

The methods adopted for the present study offer a predictable and accurate quantitative evaluation of the masticatory performance that can be used for studies investigating a spectrum of physiological and pathological conditions affecting the mastication.

The mandibulectomy patients have a reduced masticatory performance compared to healthy individuals, even after implant-supported prosthetic reconstruction. However, this protocol does not yet give us the response to the question whether, although reduced, their masticatory function is satisfactory enough to enable them adequate nutrition and quality of life. Further efforts will be directed towards the correlation between the subjective evaluation made by the patient and the quantitative analysis, to better define the adequate threshold that should be set with the present protocol to divides satisfactory masticatory performance from states in which patients are not able to masticate efficiently, which could potentially lead to oral frailty and a decrease in the quality of life.

This study presents certain limitations. Given the small sample size, although statistically significant, the findings of this study cannot be generalizable to all the cancer patients that have undergone the same treatment. Furthermore, although the masticatory efficiency is lower in cancer patients included in this research, we did not obtain patient-reported outcomes. Since this is an ongoing study, we plan to distribute validated questionnaires (e.g., OHIP-14, EORTC QLQ-H&N35, Masticatory Function Index) to new patients included in the study that can provide an assessment of the chewing function and quality of life.

CONCLUSIONS

The qualitative comparison of masticatory efficiency between test and control group of healthy patients showed different values of efficiency, attesting lower masticatory performance for mandibulectomy patients, especially in the operated site of small extent rehabilitation.

ACKNOWLEDGMENTS AND DISCLOSURE STATEMENTS

The authors report no conflicts of interest related to this study. The authors would like to thank Dr. Paolo D’Agostino (Department of Biomedical and Neuromotor Sciences, University of Bologna, Bologna, Italy) for help in editing this manuscript.

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